When setup as surrounds, the 100s will not produce sound until surround audio is sent to them. Sometimes this will be quiet and sometimes louder, but the audio is usually sound effects.

Try these setting the TV:
DTV audio setting: Auto

Input Audio Format: Bitstream

Digital Sound Output: Pass Through

I have changed DTV audio setting from DD+ to Auto. The other settings were already there. I’ll keep you posted. I’m currently watching Star Trek NG, so the audio format is very legacy and not a fair test. Should music on YouTube sound the same, whether it be from the app, or from the laptop, or does it not work like that?

YouTube is stereo. Any Sonos surrounds will emulate a very soft signal.

You should set up your surround speakers to play music  at FULL rather than AMBIENT

 However, this setting has no impact on audio that comes through the line-in. Even if it's music, you probably won't get much sound out of the surrounds. 

I suppose the obvious question is...do yo know how to setup your surrounds as Full for Music and auto revert back to ambient for TV audio when a surround track is available?