I find that pretty sporadically something will make a sound, and then mess with the audio settings of another applications.
If I am playing music in Foobar2000, and then plug in an iPod which loads iTunes. The FooBar2000 windows mixer volume will go down to like 10%. This happens pretty reliably.
Today while watching one of the Harvard Food+Science videos in Firefox, whenever I'd get a message on the Steam client that would trigger the new message notification sound, the Mixer volume for "Plugin Container for Firefox" would similarly go down to about 10%.
Does anyone have any idea what causes this, and any sense on how to fix it?