For a Mac, Audion allows playback (of MP3, aiff, or wav) at any speed from half-speed to double-speed. The "Amazing SlowDowner" also does this (for MP3, aiff, wav, or playing from CD), and I'm sure some Windows programs also do it.
   It's very useful for listening to speaking (from a lecture, interview,...) in less time, or for changing the pitch of music, for "tuning" it to a fixed-pitch instrument (or changing it to a different key) if you want to play along with a musical instrument. (it's sort of like putting a "guitar capo" on your file)

   Can speed-changing during playback also be done with AAC? Are there any programs that will do this? (for Mac? for Windows?) If not, will there be any soon?

   And will any programs change the speed during encoding? (This would be much less useful, in fact it could be dangerous! I'm just curious.)

Mike

There is a Winamp DSP plugin called Pacemaker that does it.

Apart from the tip that Pio2001 already gave you, I'm not sure what QuickTime 6 itself and/or the various plugins for QT, WMP and RealPlayer have to offer on this issue. But speed change without pitch change is implemented in the Advanced BIFS part of the MPEG-4 standard, so it can be done... err... somehow... some day...



No, this is one of the key features of MPEG-4's Binary Format for Scene Descriptions: everything can be influenced from the decoder in the "user terminal", i.e. your PC and your MPEG-4 compatible software.

Nero Wave editor is another program that can be used to change speed and pitch independently. I'm sure (though I have not tested) that other wave editors can do this too. It's useful if you want to have a pitch/speed changed copy of a song but it'd be overkill to use it for listening only IMO.