I had the same problem when I was using a tascam 414. the tape machine is recording onto the tape at really high speeds, so if u listen to the actual cassette u recorded onto it's gonna sound super sped up. the machine is the only thing capable of playing that specific tape back at normal speeds, so u need to record the audio of the finished tape onto either another cassette playing in a normal tapedeck, or just your computer where you'll end up w/ a digital version of the track w/ that tape-y goodness added to it.
it's a weird quirk of using tapes, and requires a lot of patience, but it's worth it in the end