Its a well known issue. This is the post I created for it in this forum. It has to do with Qualcomm and their drivers.
Terrible bluetooth stereo sound with the latest Cm10 builds. I found out about this when connecting my headphones. Sounds like an old radio or a phonograph. Heres the issue link:
Type Status Version Model Network Owner Summary Stars Priority
The temp workaround is to simply make a call once ur headphones are connected.
Other SG3 ROMs based on CM10 might have this issue too.
Stock ROM does not.