Guiter Hero Wii's mono sound. There is now a page up on the Activision's support site with information to pre-register for a replacement disc.
The pre-registration is available for North American customers only, and the site claims new discs will be available in "early 2008." If you don't feel like pre-registering at this time, there is also a notification by email option for receiving updates on the situation. Guitar Hero Wii's mono sound should find a nice place in "How'd the heck that happen?" History, right next to Dead Rising's teenie tiny text.