It’s a common complaint. Unfortunately, as I understand it, there is no standard for the level at which the sound should be broadcast. You can even get differences on radio stations. There’s really not much Sonos can do about it. 

Is there a TV audio output Volume adjustment? As nik9669a states, there are no standards. Some components, such as A/V receivers might offer individual input level adjustments. Unfortunately, SONOS would probably view these adjustments as “too complicated” for most users to deal with and therefore will never offer them. With respect to radio stations and online music services, each station will have a different level (that could change from time to time) and each track on the music service could require a level adjustment. Even if you are playing CD’s or records, there a level differences. Comprehensive level matching is a huge project because one must process every track in the library and then establish a common playback level reference. During playback this reference would be used to adjust the system level. Even this can sort of scheme can have some issues with tracks (particularly classical music) that have large level differences within each track. 

Ever changing volume levels drives me nuts with Sonos. Every time I change the source I am either blasted or can barely hear anything.

I understand the complexities of volume equalization, but why couldn’t a control be added to the software controller to set a settable default that is source dependent?  i.e. Level xx for Spotify, level yy for TV, level zz for Pandora, etc. etc.

In my opinion, this would be a useful feature, but I’m not optimistic. Plus, this would not solve the issue of tack to track variation. SONOS seems averse to adding options, likely because a large collection of options might make the system seem “complicated”. SONOS wants to be perceived as “simple”. While it will not help your situation, here is a sensitivity setting for Line-In.

If I have a playlist generated from multiple sources, volume level can be an issue. But if I am simply playing a single source, the volume level is constant.