The effect that I want to correct for is certainly periodic and obviously has a frequency, but it doesn't sound like the music is speeding/slowing. Instead, the music sounds like it's periodically getting slightly muffled then slightly clearer, slightly muffled then slightly clearer, slightly muffled then slightly clearer.
It sounds to me as though it may have come from a cassette tape that had been left laying in the sun for too long.
Thanks Slipstreem. Yeah, the effect is kind of similar to someone adjusting the treble up and down throughout playback I guess.I was hoping it was a common enough effect that some clever soul has built a plugin for Audacity or something. But I am denied. Harsh.Ah well - thanks again dude, and all other guys who replied. It's better to have this distorted copy than nothing at all, so I shall consider myself lucky!