That sounds to me like what i said before cookie
The bass speaker and tweeter interact with themselves,
So they kinda split themselves up if you see what i mean.
So one half (tweeter) does 80% of the voice and the bass speaker will do the other bit if the bass wasn't there then everything would be really high pitched and unlistenable (common term being "tinny")
And say if it was the other was round when the tweeter has blown:
There would be muffleness because there isnt that "80%" of the voice there so therefore you dont hear the higher tone you hear the low tone.
But cookie has rightfully said it could well be the sound card because you're saying sometimes its quiet so its the sound card might be dieing on you therefore the soundcard outputs less and less untill it stops.
Only a couple of months ago our rear right tweeter (in the "rear right speaker
in our family room blew and because its near the swimming pool (it echos through the wall
) my dad had tuned the speaker so it sounded like the others even though it was in a different "environment" so the sound was very noticble because it fuxed the whole sound up. But my dad didnt notice untill i told him (the way its setup) and when i was walking down to go for a swim it was really muffled so i told my dad and it was the tweeter.
I'll shut up now