Yup, to do that kind of audio processing requires a lot of calculus, I'm told.

Back to the original question:

Time-domain processing of audio data is something best left to more powerful chips with an FPU (such as the one on your PC). The Rio Car player doesn't have this feature, but it doesn't need to have it. Instead, do the time-processing on the file ahead of time, before you send the file to the player: Open your audio book file in CoolEdit (or some similar application), compress the time, and save it.

I think you can even open/save directly to/from MP3 format in most of the more recent versions of the audio applications, so you don't necessarily need it to be a WAV file. This will cause a slight degradation of the sound quality, but it shouldn't be a very noticeable degradation for something like an audio book. The time-domain processing will degrade the audio a bit, too, so it's not a big deal to decompress/recompress the MP3 in that case.