I too, have encountered this problem. I am running version 1.3.11 (Beta). I am recording in mono from our soundboard (Yamaha M7CL) to the channel 1 XLR input on an M-Audio Mobile Pre unit, which is connected to my computer via a USB connection. I was recording our church service today and I put on the headphones that were being used to monitor the recording and found that it switched to 'chipmunk' mode, although at the beginning, it was recording normally. It also played back the same way, normal at the beginning and 'chipmunk' mode partway through the recording. When I quit and restarted Audacity, the recording was back to normal. I then took the original recording and changed its speed by -49.5% and it sounds very close to the original recording before the speed change. I tried several different speed changes to find out which one came the closest to the original. I was surprised to find the it was not -50%, but -49.5 percent that was the closest to the original. I have tried this on two different computers and get similar results. Of course, I am now at home trying to duplicate this problem and cannot. Does anybody else have any additional information on this problem?