How do I play audio from my iMac through Sonos?

The controller is NOT a player. The analog computer audio from your Mac cannot be getting to the Connect over the CAT6, it must be going over the line-in. The Connect converts it to digital. The Mac Contoller tells the Sonos system which speakers are to play that audio. As for playing music files stored on the Mac, or any other source, the controller tells the speakers to fetch the music file from the source (which may happen to be a folder on your Mac), and play them. The music does not go through the Mac controller or any other controller. You could control all this from a Sonos mobile app instead of the Mac controller.

So, please tell us 1 more time for the record, what " player " was being used in the simple 3 piece configuration listed above ? A.K.A. My test bed
The PLAY:5 obviously. It's the only player.

The crucial thing to grasp is that no Sonos controller, whether on a Mac or a mobile device, processes audio. It issues instructions. The players (speakers) get the audio direct from the source, as instructed. Even if the controller happens to be on the Mac and the music files happen to be on the Mac, the controller is still not "playing" the music.

For the analog audio sent out to a line-in, your Mac is providing the source via its sound card - it still isn't the Sonos controller app that is processing the audio,

To add to my previous comment, this is SO fundamental to the way Sonos works (multiroom, multi-source, multicontroller, not through the device) and is SO fundamentally different from technologies like Airplay and Bluetooth, that when you have grasped it is blindingly obvious why it isn't a simple software tweak to play computer audio wirelessly.

I think we all understand players are players and controllers are controllers.So, please tell us 1 more time for the record, what " player " was being used in the simple 3 piece configuration listed above ? A.K.A. My test bed

Actually, it seems you do not understand at all. In your test bed, the Play:5 is the player. Period. It is the only unit of the entire Sonos chain which fetches, receives, processes and outputs the audio stream. No audio stream passes through the Mac or the Sonos controller at all. None, zero, nada. There is no capability for the Controller on the Mac to play, decode, process, push out, pass through, or even recognize what the audio stream is, outside of the Play:5 sending the track meta-data back to the controller.

So now do you "understand" why switching from a simple controller with smart players that fetch audio to a smart controller that processes audio to "push" to a player that simply plays an audio stream is much more than just a "few lines of code"? If you do not, then I don't know what to say to clarify any more.

Umm... Wow ! I'm glad I purchased and thourly tested a test bed before making the substatantial investment required to install this system.Had I depended on the forums I certainly may have reconsidered and perhaps even seeked another solution, non the less,so to be very clear,I choose to use my Sonos speakers simple as designed to be used, an amplified speaker, simple as that.The Sonos app and its " as you all position limited capabilities " is of very little use to me personally as a music service system.I think one might find it very difficult to push/route data to 28 separate players and have them all stay in perfect sync.
Sorry I wasted your time expressing my stupidity,ignorance,or there lack of.

The Sonos system actually syncs music perfectly with up to 32 players. Part of that perfect sync is the design architecture which fetches music from the source, instead of it being pushed from a computer or device.

And if you are simply looking for an amplified speaker, then you picked the wrong product. There are plenty of standalone amplified speakers that will fill your needs.

Yes, it has been a waste of time, clearly, and I shan't be wasting any more here. But @jgatie - @jonf1965 says he already has a substantial Sonos system. He just doesn't understand it. I no longer care, and maybe none of us should.

I'm also bemused. With 28 players this must surely be a (mostly) happy Sonos customer. It just seems there's a gulf between how he thinks the system works and how it actually does so.

I'm also bemused. With 28 players this must surely be a (mostly) happy Sonos customer. It just seems there's a gulf between how he thinks the system works and how it actually does so.

I don't think he has 28 players. I'm pretty sure he was testing out the Play:5 and the "28 players" was either his plans for the future, or yet another "I would have bought $10,000.00 of Sonos stuff it it only did this! Sonos is losing my money!" posts.