Good one! As I recall, they constantly belittle director Mary DiBergi (Rob Reiner) for only focussing on the things that went wrong. Then, whenever one of the songs is being performed, one bandmember (Bassist Derek Smalls/AKA Harry Shearer?), with an almost reverent tone, recommends that we just listen to the performance...
An inspired idea. Reminds me of one of the commentaries in the Freaks & Geeks DVD package: Three actors do "in character" commentary for one episode. The characters were the gym teacher, the counseler, and the math teacher. Not nearly as successful as the Spinal Tap commentary, but still a nice little extra, and for all I know, may have been an homage to it.