It’s hard to answer your question directly…

SonosNet is a parallel version, so to speak, of Wi-Fi. It’s a radio signal, subject to the same physics as your Wi-Fi signal is, including wifi interference. 

It’s not clear from your question, however, what you mean between the Sonos App, which runs on your speakers, and Apple Music, which could be the included part of the Sonos App, or it could be running from an Apple device and being sent to the Sonos speakers separately, via AirPlay 2. 
 

Based on your final statement, an increase in ‘spottiness’ as you increase the number of speakers in use is a great indicator of a small level of wifi interference. The bandwidth necessary, as you increase the number of speakers in use, also increases. You may be at a ‘tipping point’ that your network just starts choking on as you increase the number of speakers in use. And certainly if using AirPlay 2, I would recommend sending one stream to a Sonos speaker, then allow the Sonos controller to do the grouping. 

Thank you, Bruce. 
 

I believe I may have used the term app in this case incorrectly when I said Sonos app. But was referring to the Sonos Controller app installed on my iPhone. 
 

My observation has been, when changing nothing else besides the method of initiating a stream, I experience different results. If I play music from Apple Music, I encounter dropped speakers much more frequently than from playing Apple Music directly from the Sonos Controller app. 
 

Given that, I became curious whether the 3 Boost devices I have on my system are utilized when I play a stream using AirPlay 2. Or is it just AirPlay 2 doing a horrible job, if that’s possible. 

BYW - my system has 11 speakers.  Two are the portable Move speakers which don’t use Sonos Boost’s isolated network. 

BOOST uses a private mesh network, “SonosNet”, to allow SONOS units to communicate with each other and music servers. Airplay 2 is constantly using WiFi. Likely, your phone is encountering some WiFi issues. When SONOS players are directly managing music play, the controller device can be shut down.

The very latest SONOS units MOVE, ROAM, ERA100, and ERA300 will not use SonosNet. MOVE and ROAM are WiFi only. The ERA units are WiFi unless a network connection is available through a USB-C accessory.

Yea, it gets complicated pretty quickly, finding the right terms that everyone agrees on. 

The BOOST devices only help the Sonos speakers communicate with each other, they are not ‘WiFi extenders’ that AirPlay 2 latches on to. 
 

As I suggested, the more AirPlay 2 streams you’re sending from a device, the greater the bandwidth used by that device. Not as big an issue on a Mac, but it might be on an iPhone. It’s why I always send a single AirPlay 2 stream to my Sonos, and then let the Sonos system, not my iPhone, do the heavy lifting to distribute it to other ‘rooms’. 

If you stream music to a group of Sonos speakers via the Sonos App - one speaker is selected as the Group Coordinator and it receives the music, in your given example, direct from the Apple cloud music servers - the Sonos controller App is not a music player, it’s just a remote control.

On the other hand, the native Apple Music App is a music player and it gets the music from the cloud onto the mobile device and then plays the music to the Group Coordinator - the fact that the stream goes via the mobile is not always helpful, particularly if that mobile device is roaming around the premises and so it may encounter WiFi interference and more often than not, such mobiles perform other tasks in the background too, like downloading updates, receiving messages and mail and numerous other things.

So that’s why using the Sonos App is the better, and often more-reliable, option.

You mention having three Boosts - if you want to know if they are being used effectively in your system then using a web browser navigate to http://IP_Address of Any Sonos Speaker]:1400/support/review and select to view the ‘Network Matrix’ and if you look closely you will see the Sonos devices that are in use/connected to each of your Sonos Boosts, or if they are not in use.

Hope the above assists.

Thanks, Bruce, Buzz and Ken! I think I understand now. 
 

If I want to stream outside of the Sonos Controller (S2), I should probably stream to one and add the other speakers using S2. I mostly stream from my Apple TV, but occasional from my iPhone. 
 

Appreciate all of your input!