There are plenty of algorithms out there which allow you to change the speed of the recording without changing the pitch. Some of them are very fancy, and they have to do a lot of math (calculus, actually, IIRC) to make it sound natural instead of electronic-y.

The best versions of these algorithms are used in professional audio editing packages, and the details of each are kept trade-secret because it's a selling point for some of these packages.

Note that the converse is also something that is done often in these packages: Changing the pitch without changing the speed. Same principle, similar algorithmic challenges, just along the other axis.

When the pro packages do this, it's not usually in real-time, it requires that they churn pretty hard on the file to produce a decent result. And you can only go so far off of the root pitch or time before it starts to sound strange.

I agree, this would be good for audio books. Not sure if the empeg's got the kind of CPU to do this well, but perhaps a lower-quality version would be a nice feature.