Rockbox

The archos OS manages to play samples of the sound
during fast forward and rewind.

This would be great in RockBox as well. If this is
difficult, a much
easier but still useful form of audio cue would be a
series of
"clicks" that indicate the current FF speed.

eg: click once for each 5 seconds of cursor advance.
For accelerating FF, this results in an accelerating series
of clicks that eventually merge into an apparently
continuous tone of increasing pitch as the FF accelerates.
(a sound sort of like that made by playing cards in the
spokes of a bicycle wheel)

This would help considerably when cueing around in an
hour-long
MP3 (books on tape, NPR programs, etc.) while unable to
view the screen (exercising, driving, etc).
.