If I could add one improvement it would be to allow playback at 1.3x normal speed with normal sound at 1.3x.

This idea is not new. The only reason I also have a Panasonic DVR (not TiVo capable) is that it has the 1.3x feature. This feature allows the efficient viewing, with normal sound, of any material - but is especially useful for such programs as news and talk shows. The faster rate/viewing saves time.

Microsoft Media Player currently has this feature at 1.4x. Some Sony Betamaxes (remember those?!) had the feature at 1.5x - a little too fast.

Just press the button with a picture of the Chipmunks on it. :P

First time I heard this suggested, someone said there was a technical reason why it wasn't practical because of the way compressed sound is encoded in MPEG. But I don't know if that's really true.

I doubt it. I have a relatively recent version of PowerDVD on one of my computers which is able to play at 1.x with sound. (Basically the first "click" of the jog dial.) Older versions didn't do that. But a PC uses a software codec (i.e. easily ugradable/replacable) and has lots more computing power than a TiVo.

My guess is that it's a limitation of the hardware MPEG decoder in the TiVo, which probably explains why some of those other CE devices mentioned in the OP are able to do it. Their decoders are probably either better or different.

"A limitation of the hardware MPEG decoder in the TiVo" is exactly the kind of "technical reason why it wasn't practical" I had in mind, actually.

Cool. From the way you worded your original statement, I thought you meant it was an inherent limitation of the MPEG format.