It sounds like the on-board sound card is on its way out, sound cards are pretty cheap. You should be able to pick one up for less than a tenner, of course that depends on the quality of the sound card that you want.
I've come across this problem before with soundMax onboard, the solution was to open SoundMax volume control panel and reduce the wave volume slider from 100% to about 75% (or lower) and that sorted it. Hope this works for you.
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