I've now found a related issue when I play CDs, particularly the Phantom of the Opera soundtrack (borrowed from a friend). Track 1 is fairly quiet, so I turned up the speakers, and had that playing in the background. Track 2 ("Overture/Hannibal") is somewhat louder... So, there I am, typing away, and suddenly I get the organ music of "BUM! BA-BA-BA-BA-BOOOM!" crashing out; almost made me jump out of my chair!
This has been a public service announcement :)
(As a side-note, I think that the "Overture" half of the track is far better than the "Hannibal" half.)