1) On the iPod set the volume limiter to maximum.
2) Download some .p files (ZX81 files) - eg from here:
3). Now you need to convert the .p to an audio file. I use a Mac, but the program I use (ZX81 Tape Converter) is java, so will run on anything. You the programme here:
4). Open the .p file, play it once (which converts it) and then 'Save as' to convert it to a .WAV file. If you just try to save without playing it won't work.
5) The files it creates are too quiet to load, at least with my iPod, so now open the file in Audacity (http://audacity.sourceforge.net/) and from the effects menu amplify it about 5 times.
6) Now save it, making sure you save it as a WAV and not an MP3.
You should now have a WAV that you can sync to your iPod, and load just as if it was a ZX81 tape program.