Audio shorts out after prolonged use, what's wrong?

Specs : I have a 15" Unibody MacBook Pro from mid-2010; 2.4 GHz Intel Core i5 with it's original 4.5 GB 1064 MHz DDR3 memory. I had the logic board replaced under apple care 2 years ago do to graphic artifacting, so the logic board is younger than the laptop as a whole.

Issue : My laptop seems in working order, save for the audio.

After about 15 to 30 minutes of use, the audio will start shorting out. I can get the audio to stay on longer if I stick something in the USB port and wiggle it around a little or by putting the laptop to sleep and waking it right back up, but that only postpones the inevitable. Eventually the speakers will let out this loud, cantankerous static noise (even if it's on mute, always the same deafening volume) and then go completely silent. There's no getting the audio back after that without restarting the machine.

My fear is that it's something to do with the logic board. I had researched issues with the left i/o board, but am not sure if my unibody Macbook Pro even has one.

I'm hoping against hope that it isn't the logic board. Last time I went to the mac store, their diagnostics seemed to indicate the logic board was healthy but I don't know what else could be causing this ear-sore.