Sound and heat when lid is closed - MacBook Air
Playing loud music? I doubt it. The sound probably won't sound as good (as it'll be muffled), but it'll work.

Heat? Maybe. I have an older MacBook Pro that gets its airflow in through the speaker grilles and out through the hinge. I don't run it when closed for that reason, but that is supported (it wasn't on my even older iBook G4). If you can wake it up with the lid closed, it'll probably work fine. Try it a few times and see if the screen is uncomfortably hot after a half hour or 45 minutes.