I fully second that request. I came to search for exactly this request, but I think it can be even used for more, just a second track that can be used (and labled) freely by the streamer. Might be just without music, without sound alerts (to not wake the sleeping lurkers) or, maybe for events, with simultaneous translation or audio description for the visually impaired.

The implementation would be relatively straightforward, in the settings (maybe later in an overlay / own switch), the user can select the active audio track, and the stream is changing to the source with that track, similar to switching between different qualities / resolutions. In the end, it's just a different stream source endpoint.
I don't know how the backend is implemented right now, I hope it's not leading to a resource problem to provide the double number of endpoints per stream; the external traffic, however, shouldn't change.
For the streamer, it's just updating the checkmarks in OBS in the same way as done for the VOD. (And yes, in this implementation, the second track would be selectable only in live stream, not in the VOD. Would be totally fine for me.)

I came to say just this !!!
If I like the stream and don't like the music.. im out..

So I feel bad when I play music as I know that if people don't like it then they are off or will just mute the tab and lurk... which seems to hurt the stream more now than previously..
Side note I have have 70+ accounts in stream and twitch only shows I have 30 so lurkers don't count anymore.
which if they don't like my music then am I forced to not play any as the only option? or suffer the viewer hit.

This would be a HUGE bonus and allow for more engagement over all. I 100000% agree this needs to be an option.
a separate audio slider for the music. I can separate it from the VOD so why not from the live streram... its not a costly thing to stream audio...