what I really hate is when the room is set to use mic's you get stuck with some really anoying music. At first it wasn't so bad but good god the music is now annoying. I wish there was an option to disable the "online room" music but keep you're own custom music.
IMHO there shouldn't be ANY music in the lobby so you had some quiet time to her people if you aren't using a mic. Not to mention when you do use a mic that the same same music is piped back to you, now that's really annoying.
But yes while in game (without having to go back to options and back out of an online session or endurance race) you could adjust volume of music.