Are you using GoogleHangouts rather than Roll20 proper to play your games? In Roll20 (I'm not sure about GoogleHangouts), every single time someone links into a campaign, they're prompted with an Adobe Flash Player window that asks if Roll20 has permission to allow mic and sound. (see below attachment)
Every player can manage the master volume of their music by going to their My Settings tab (The tab with the gear icon). Towards the bottom of that tab there's a slider called Master Music Volume Control which they can slide down to 0% and essential mute the BG audio.
Also, under the My Settings tab, at the very bottom are three drop down menus:
Video + Voice
I want to Broadcast to others: [drop down menu]
I want to Receive from others: [drop down menu]
Video/Player Avatar Size: [drop down menu]
The first two drop downs you can select: Video and Voice, Voice Only, Nothing (Both Disabled)
The third can either change the size of the user avatars or remove the avatar's (if no one's using video) completely.