Reduce idle consumption energy level (currently around 5W)

This is a significant issue because Sonos and other manufacturers have low-cost options to enable a very low-power standby mode that meet all requirements for home area networking and user experience of a wireless, multi-room home audio system.

(Along with Ed, above) I encourage Sonos and Sonos users to look at Apple TV as an example of an always-ready, WiFi multimedia device that uses less than 1W in standby mode. There's no reason my Sonos Connect should use 5-6 Watts in standby mode (I measured using a Kill-a-Watt power meter. My Play:1 also uses 5-6 Watts after being idle 3 minutes, which is more than the 3.4 Watts published by Sonos) https://www.apple.com/environment/reports/docs/AppleTV_Product_Environmental_Report_2012.pdf

The Median number of rooms in a U.S. home is 4 to 6. Five Sonos devices running 5 Watts for 20hrs/day would use roughly 200 kWh per year. At $0.10/kWh, that's $20 a year. NRDC published a related report on the hidden costs to consumers, public health, and the environment of always-on device architecture, called "Lowering the Cost of Play." http://www.nrdc.org/energy/game-consoles/lowering-the-cost-of-play.asp

Certainly, any given user has a choice whether they care about standby power use. But, when the costs are hidden and the manufacturer fails to make simple choices to be more efficient, we all pay the price by more fossil-fueled power plants running around the clock to fuel "energy-stupid" devices. 
No, I put a comma after "always-ready" in that phrase. I can't run while I'm asleep, but I can be woken from sleep by a smoke alarm and run out of the house in a reasonable amount of time.

In brief, good networks shut down all but one WiFi transceiver in the network (questionable whether that is even necessary in Sonos' case) and rely instead on low-power communication for mesh networking  (such as those that employ ZigBee specs) to maintain awareness in a sleep or standby mode.

Apple knows that it doesn't make sense to maintain a WiFi mesh network when a high data rate is not needed. They've been using wake-over-wireless technology for years because it's cheap (ball park of $1 per unit?) and makes technical sense and they want to compete well with smart companies.

This is a significant issue because Sonos and other manufacturers have low-cost options to enable a very low-power standby mode that meet all requirements for home area networking and user experience of a wireless, multi-room home audio system.

(Along with Ed, above) I encourage Sonos and Sonos users to look at Apple TV as an example of an always-ready, WiFi multimedia device that uses less than 1W in standby mode. There's no reason my Sonos Connect should use 5-6 Watts in standby mode (I measured using a Kill-a-Watt power meter. My Play:1 also uses 5-6 Watts after being idle 3 minutes, which is more than the 3.4 Watts published by Sonos) https://www.apple.com/environment/reports/docs/AppleTV_Product_Environmental_Report_2012.pdf

The Median number of rooms in a U.S. home is 4 to 6. Five Sonos devices running 5 Watts for 20hrs/day would use roughly 200 kWh per year. At $0.10/kWh, that's $20 a year. NRDC published a related report on the hidden costs to consumers, public health, and the environment of always-on device architecture, called "Lowering the Cost of Play." http://www.nrdc.org/energy/game-consoles/lowering-the-cost-of-play.asp

Certainly, any given user has a choice whether they care about standby power use. But, when the costs are hidden and the manufacturer fails to make simple choices to be more efficient, we all pay the price by more fossil-fueled power plants running around the clock to fuel "energy-stupid" devices. 
In  my case, everything is on Wired Ethernet, so actually I'd love to be able to turn off the Wifi to save that power all the time, and as noted above, simple WoL would let the unit stay in sleep most of the time. Even in the mesh case, wake on wireless would let the units mostly be asleep when not in use, and wake up to create the mesh if needed to play something. For me and I suspect most people, most of the time I'm not playing anything.

The SONOS power consumption is totally crazy and certainly not in fasion. This goes for the environmental and the cost aspects both. I have twelwe SONOS speakers in my home so far, including four of the really thirsty Play:5. None can be switced off by me manually when not in use, since they are all wall mounted - and also, that is not intended by SONOS. I eagerly expect a solution with a real sleep mode. SONOS seems so proud of the at-once ready to play ... but honestly, who cares? Waiting for instance twenty seconds would be totally ok if we could get rid of the actual power consumption craziness. Fixing this can only go to slow. And it should include existing SONOS speakers - hence be a software update-based solution.

Count me as one who would not accept waiting 20 seconds.  I'd rather pay for the extra electricity.

Off cource the sleep mode would be optional, so your comment is irrelevant.
Your opinion makes sense if ypu have a couple of speakers, but I have twelve speakers, and I've calculated yearly standby expence of aprox 200 Euro for these SONOS alone. That's totally ridiculous.

Agree. Any comment aiming to keep status quo is irrelevant by default.

Honestly, I feel it would be easier if Sonos could provide a way of reminding the previous status before shutdown, allowing those 20 secs to be even less annoying.

It's not painful to add power switches, then. That's what I do with mine, I'm not bothered by the boot time, only by having to reconfigure everything each time.

Nobody should have stand-by devices, regardless of consumption, it's a matter of security.

It's not painful to add power switches, then. That's what I do with mine, I'm not bothered by the boot time, only by having to reconfigure everything each time.

I must admit for me it turned out to be painful. I installed radio-controlled plugs for my Sonos devices but only used them for two weeks or so. After that I considered the loss of convenience higher than the loss of money due to idle consumption (though being located in Germany with very high energy prices of 25-30 ct/kWh). It's one of the fantastic features of Sonos that you can switch your music on from everywhere in your house by just taking your mobile out of your pocket. And it starts immediately. You don't have to look for a remote-control, you don't have to stand up or to wait, you can resume listening exactly were paused earlier...

Hence I'd definitely appreciate less idle consumption by the Sonos players. A solution should however not undermine the particular character of the system. 

And, honestly said, I was aware of the consumption when I bought my players. And I'd presume most of us were. At least here in Germany it is merely not possible to inform yourself about Sonos on the web without being pointed to that issue by recessions, blogs etc. Thus, it's your choice like it was mine. And I do not regret it. 

I instead regret the choice because I don't like the sound quality, but also realized that finding an alternative is not easy.
I really hoped to replace my Sonos speakers with the new Bose SoundTouch II line and ordered the flagship 30 just recently for a test, but instantly returned it... far better sound quality than my Play:5s, loved the remote and display, but overall barely usable (c'mon Bose, Spotify Connect only, are you kidding?) and I also missed the stereo pair... not mature yet.
I was thinking to replace my Play:N speakers with Sonos Connect devices and quality speakers, but I found the price total nonsense... how can a Connect be priced higher than a Play:1?

Exactly

I agree, an on/off switch or power down mode is needed. I just had my Play5 replaced because my the connector broke from pulling the plug in and out so many times. Now I have an on/off switch at the Receptacle.
An on/off button would only help some. My twelwe SONOS are all wall mounted in 2½-3 meters height, plus they are all with cut off cords, hidden and plugged directly into lamp sockets just behind the speakers - just as it should be.