Sorry, I was searching for information in X-IVAP and stumbled across this thread - Why not buy a cheap USB headset and use that? One good thing about XSB is it allows COM1 and COM2 to be independently set to different audio devices. So, I have COM2 on my speakers along with the aircraft sounds, useful for monitoring chatter during cruising, and COM1 on my USB headset which I can turn up and down independently of the speakers. Get a headset with simple in-line volume control and you can set it really quickly and easily while flying.
Of course, if you're already using a headset for all your sounds then that idea goes out the window...