Apart from the expensive turntable- and computer solutions described above, here's what you can do with an old audiocassette:
- Rewind the tape. When you lay the cassette down with the tape-head holes towards you the full spool (with all the tape) should be on your left after rewinding.
- Unscrew the plastic case.
- Carefully take out the small spools holding the tape and lay them flat on the table. Keep the same flange of the spools up. (That is: don't flip them when you take them out of the case)
- Swap the spools in such a way that you do not flip them and be careful not to fold the tape. The full spool should now be on the right, but the same side should still be facing upward (Otherwise this would just be a cumbersome way of playing side B now, wouldn't it).
- Mount the spools back in the plastic case. Take care to properly lead the tape past all the capstan- and pinch roller holes as it was before you took it out.
- Carefully rewind the tape so that it rolls on the left spool correctly. Do it by hand if you're patient enough.
- Play and enjoy your Satanic message of choice!
Note that you are now playing the backside of the tape. Cassette tape usually isn't coated on the back so this is no problem. It might sound a bit dull though. Suggested listening: the breakdown in Vision Thing by the Sisters of Mercy has a nice backward sample. For some truely evil stuff (*sigh*) listen to the intro of Satan spawn: the Caco-daemon by Deicide.
Disclaimer: I will not be liable for any damages resulting from accidental invocations of Cthulhu, your deceased grandmother OR breaking your family-tapedeck.