Does everybody gain access to these expansions, too?

Also, if I play multiplayer with someone who does not own the soundtracks, do I still get to listen to them?

ShadeDev wrote:

Does everybody gain access to these expansions, too?

Also, if I play multiplayer with someone who does not own the soundtracks, do I still get to listen to them?

I believe since soundtrack dlcs add a hero, they count as adding content to the game so to answer your question, yes they do.

What about the inverse?  Do I lose access to the additional music when playing with someone else who doesn't own the expansion?

Everybody use DLC list of the person that hosts. So it's not like you lose it, you just don't use it, if host doesn't have it.