Sounds like the batteries weakened in the course of recording.  The machine slows down and, when played back at a uniform speed, speeds up.

Steve Smolian

-----Original Message-----
From: Association for Recorded Sound Discussion List [mailto:[log in to unmask]] On Behalf Of Grant, Tyra
Sent: Friday, February 26, 2016 11:43 AM
To: [log in to unmask]
Subject: [ARSCLIST] Cassettes that speed-up---what's up?

We're transferring a collection of personal cassettes---interviews---mainly from the 90s.  
A few start out okay then slowly speed-up so people sound like The Chipmunks.
Then, some are okay on one side but the flip side is speeded-up.
What's up here?  
We'd appreciate recommendations re: the best way to handle these in order to get something listenable.

Tyra Grant
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University of Kansas Libraries