I was running into this too and it was driving me nuts. What fixed it for me was this:
1) Once Parallels Desktop was launched, select the Configure option from the Virtual Machine menu
2) From the Sound menu, check your Output option. My Output had 2 "Default" options, one at the top of the list, the other at the bottom.
3) Select the "Built-in Output" option
4) Lock the Configure area so the VM can't make further changes
5) Restart the VM
It's been quiet ever since I've taken these steps. I've also noticed the 2 'Default' options are back to just 1.
Hopefully this works for you.
Early-2011 MBP (16G RAM, 256G SSD)
OSX 10.8.5
Parallels Desktop Build 8.0.18608
Win7 VM
Last edited: Oct 19, 2013