In OBS Studio and Streamlabs OBS, the streamer is able to Stream on certain Audio Tracks, and Record on certain Audio Tracks. As more and more integration with the OBS' and Twitch comes alive, this should be something that can be done. To simply put it, as a Streamer, I could put my Microphone on Audio Track 1, my Gameplay on Audio Track 2, my Music on Audio Track 3, my Voice Comms on Audio Track 4, and my OBS Alerts on Audio Track 5. Then, in the Twitch Video Player, give the Viewer the option to select which of the Audio Tracks they would like to listen to. If they only want to hear the streamers voice, then they would turn off Audio Tracks 2 through 5, and keep Audio Track 1 open. If they wanted to listen to the game as well, they would enable Audio Tracks 1 and 2.

As a streamer who uses Voicemeeter Banana and VACs to split audio up, who also runs Song Requests from time to time, I know this is 100% possible and useful. Possible because there would be 5 different audio channels streaming to the website separately, so allowing a user to toggle which ones they want to listen to should be fairly simple. Useful because if I am running Song Requests on my stream, and someone really wants to watch my stream, but doesn't care for the background music, or wants to listen to their own while lurking, then they can have that ability.