As I mentioned before, for music and sound effects we just play them through USB-connected PC speakers and let the microphone pick it up. These are Sean's fancy speakers with infrared remote and a subwoofer. I usually adjust the volume with the remote for convenience, although I should really find a better way.
Usually both speakers are facing away from Sean and Jen and the mic, towards where I sit. Up until Friday I turned one speaker towards them and left one facing me so the infrared remote would work. I watch the VU meter on the video encoder to gague where I need the volume to be, but it often seems a bit loud to Sean and me, and it isn't coming across as well as I'd like on the microphone.
This week I realized their monitor (doubling as a stand for camera one) was between the speaker and microphone, so I moved the speakers forward so both of them face the microphone, and the IR-receiver speaker is also tilted towards me so the remote works. I think the music sounded noticeably better on this week's recording, and the VU meter looked like it was picking up enough volume without Sean or me complaining about the in-studio volume.
I guess that's so simple it's stupid, but sometimes you don't notice the blindingly obvious until you look at what you're doing and ask yourself if something could be done better.