Utter noob, just needs a place to start

I am utterly new to processing audio. I am willing to read the tuts, but have no idea where to start, or what language to use to search for a solution to the problem I"ve encountered.

I’m converting old (15+yrs) audiobooks on cassette to mp3. I’ve successfully converted three full books (about 15 tapes each) and the results have been brilliant. Made it sound like I knew what I was doing (heh!). The cassettes for the fourth project, though, have not aged well. They don’t play (or at least they don’t sound as if they play) at a consistent speed. At times the reader’s voice is pitched much higher than at other times. He never reaches chipmunk stage, but gets close, and at other times sounds l-o-w and s-l–o—w. The change is frequent and extreme enough that it becomes a distraction (and a little comical in some places, where the timbre doesn’t quite suit the action of the text).

Might someone with more experience tell me, please: 1) if this is something that I can correct within Audacity and 2) what the correction is called - what is it I am trying to ‘fix’?

Thanks for taking the time to read - hope someone can put me on track.

It’s not something that you can “fix”, though if all else fails you may be able to improve it a bit with Audacity.
The better approach is to see if you can get the tape to play better.

The likely cause is that the tape is deteriorating and the magnetic surface is turning to dust and/or becoming sticky. The tape may also be stretched and will probably have become quite brittle, so you will need to handle it carefully.

The first thing to do is to wind the tape fully through to the end and back again. You can wind it most of the way using Fast forward on your player, but stop before you get to the end and wind the final part by hand with a pencil through the appropriate wheel. (winding it to the end with fast forward could cause the tape to snap when it gets to the end and they are a bit tricky to splice back together).

The next thing to do is to clean the cassette player - use a soft paint brush or blower to clean out loose dust (don’t blow - spit in the mechanism is not good and inhaling the dust is probably not a good idea either). Then use either some cassette head cleaning fluid, or pure alcohol (for example IPA) or a mix of alcohol and distilled water with cotton buds to clean all of the surfaces where the tape touches. Pay special attention to the “pinch roller” and the “drive spindle” - these should grip the tape while it is playing and feed the tape past the playback head at a constant speed - you will probably get a lot of muck off the pinch roller. Avoid splashing the solvent onto any part of the case as it could damage plastic case parts. Also take care not to leave bits of cotton bud in the mechanism

Allow the mechanism to dry very thoroughly.

After doing this I like to play a couple of minutes with a “tape head cleaning cassette”.

If you cannot obtain a cleaning tape you can just test the mechanism with a good condition tape (a blank tape is a good idea if you have one) just to make sure that everything is dry and not sticky and is not going to chew up an important tape (don’t use an old dusty tape, or one of your favourites).

Now try playing the cassette again and recording it.

Thanks very much for taking the time to read and reply - I’ll give cleaning the player a go. (I very nearly remember the days when I kept those head cleaning tapes around!)