From my experience it works on all audio. You can switch it on and off and you’ll notice a difference.

If you notice a difference it’s entirely psychological. As it says on that option it’s a “setting to override the preset for Line-In”.

https://support.sonos.com/en-gb/article/adjust-line-in-audio-compression

Well we’re getting off track here my original Post was that they lowered the threshold of audio compression much like Bose does, and other companies I hope future firmware updates, corrects the problem.

How are you able to determine that’s the actual issue?  It sounds like this is your best guess, and you haven’t actually talk to Sonos or done anything to determine that’s what’s actually happening.

My ears , haven’t failed me, nor my wife’s or my sons or my friends they’ve all noticed a change like I said I hope a firmware update helps

I’m sure there’s Sonos employees part of this community. they might no changes?

I’m not doubting your ears,  you claimed that the reason for the audio issue was due to compression, and you seemed confident in that.  How do you know the audio issue is due to compression?

Are you familiar with audio compression and how it works i’m sure you’ve experienced it

if not, go online and look it up

So you’re talking about audio compression, not data compression, and your evidence is just your ears.  Got it.  I really don’t think you can assume that Sonos engineers intentionally compressed the audio, and this isn’t a bug that needs to be addressed.  I would definitely contact Sonos and send them a diagnostic.

Compressors are one of the most important effects to understand when trying to get recordings that sound professional and radio-ready. When used properly, they can make your tracks sound punchy, balanced, and loud, while also injecting coloration and tone. 

But over-compressing can squeeze the life out of your recordings. So having a good grasp of the basics will ensure that you are getting the most out of this essential tool.
 

Threshold

The threshold is the level at which the compression effect is engaged. For example, if the threshold level is set at - 10 dB, only signal peaks that extend above that level will be compressed

All I can say is they made a change and I noticed it so I rest my case it’s disappointing, but maybe future firmware might help

When there was an actual sound change in a Sonos device, the thread had over 150 posts in the first 24 hours.  This thread has one person with the problem in 3 days.   I don’t care what case one is resting, the evidence isn’t pointing at Sonos. 

Could you perhaps be mistaken about the dynamic range compression here @Tulo, in a similar way that you were clearly mistaken about the line-in data compression changing your streaming music audio, as that Sonos App setting has no effect on streaming audio. That feature applies only to line-in audio?

Have you checked all your Sonos App ‘room’ settings and ensured you, or someone else in your household, have not enabled ‘night mode’ when listening to TV audio, or reduced the volume level of your Sub, or changed your room EQ whilst listening to music audio etc?

The Arc, surrounds (Era-300s) and Sub (gen3) setup here with an LG C9 TV sounds fine to my ears and to the Wife’s ears too - We’ve not noticed any sort of significant sound change by the recent S2 firmware updates. 

Our Sub Bass output when watching the movie John Wick 4, for example, still sounds very punchy, as do our streaming music services (Amazon Music HD/UltraHD). Certainly the start of the John Wick movie is more than enough to have our neighbours complaining, even at a 35% volume level with ALL settings set to default... it still rattles the glasses in a cabinet we have next door in our dining room.

Yeah my arc went down in volume and lost punchy bass, I can’t even run true play as it’s not working on iPhone 12 since the update. Before when I turned of Sub the arc took over the bass but now it’s stays the same with or without the sub on, literary not much difference than my Sony tv speakers. I have it only 2 months but will be returning have enough this crap

If you and any others in your household cannot tell the difference in audio output when the Sub is enabled/disabled, whilst playing a variety of music sources, or surround sound movies with DD 5.1 codecs etc; then you really ought to submit a Sonos diagnostic report and contact Sonos Customer Care as there is quite clearly something wrong in that instance. I have no issue hearing a difference when toggling the Sub ON/OFF and my daughter/son-in-law at their Home say they can hear a clear difference too and both are happy their SUB is working correctly with their Arc/Sony TV. 

15.9 firmware has restored the dynamic range thanks, Sonos