This is typically caused by recording when the batteries are getting weak. Low battery voltage can cause the drive motor on a cassette deck to run slower than normal. When the tape is later played back with fresh batteries or using wall power, the slow-recorded material playing at normal speed sounds faster than usual, hence the Mickey Mouse effect.
If the tape is consistently slow, all it takes to correct the problem is a cassette
deck with variable speed playback, or pitch control. But as the batteries got weaker, the
recorder speed may have not only been slowing down, but also getting erratic. If the speed
was going up and down there's little that can be done to salvage the recording.