I get a vaguely similar problem with external speakers with the 12"
After about 4 days of uptime, they get very, very distorted when I change the system volume with the keys on the keyboard... Works fine with a software volume change in OS X though... seems quite strange. The problem doesn't effect internal speakers, or headphones.
It also happens on occasion when the modem is used with 'sound on'...
Anyone else experienced this?
A restart cures all however, so I'm happy...