I'm not a expert or anything,
But it could be a little resistor has blown up and when your try and boost up the Db (volume) it has to much cutting the power.
I'll ask my dad as he's a hi fi freak
<offtopic>Hey I'm msg plus freak
and my dad is a hi fi freak
</offtopic>
EDIT: He said that it is probally to do with the sound card beacuse there might be a shortge in the sound card so when it try's to send more power it just cuts them off (switches off)
I've probaly explained that wrong
EDIT AGAIN: This happens to me on my power amp it like sorts of crackles turns off then comes back on, usually because it is either damp hasnt been used enough, or too much. Solve by going up and down a lot (if you have a external power switch) and it should resolve itself
/ 
