I got the audio working on my MobilePro 800, sorta.  I held down the
Brightness Down button while hpcboot was finishing right before the
kernel takes over.  WinCE keeps the amplifier on to make its stupid
condescending Fisher Price keyclick sounds.  It makes a noise like,

  CliCliCliCliCliCliCliCliCliChhshhershshrhhghK

then silence.

audioctl says it is capable of 16-bit mono.  madplay can play a mono
MP3 at 44.1kHz, and I can listen to my j2.com voicemail with 'toast'.

AFAICT there is no volume control, and besides being mono the sound
quality is very bad---noisy for example, you can hear the machine
thinking.  You don't want to listen to music this way, and it says
it's half-duplex so you can't use it as a IP telephone either, but for
playing voicemail-to-email it's good.

If you are wondering how fast is the MobilePro 800 for playing MP3s
with mad, here are my results:

Script started on Thu Feb 27 15:15:00 2003
\h:\w$
\h:\w$
\h:\w$
\h:\w$ time madplay -m -v --output=raw:/dev/null sunspot.mp3 ; time madplay -v --output=raw:/dev/null sunspot.mp3
MPEG Audio Decoder 0.14.2 (beta) - Copyright (C) 2000-2001 Robert Leslie
     Title: sunspot
    Artist: Banco de Gaia
     Genre: Other
 00:06:59 Layer III, 128 kbps, 44100 Hz, single channel, no CRC
error: frame 16041: lost synchronization
16041 frames decoded (0:06:59.0), -0.8 dB peak amplitude, 0 clipped samples
  299.84s real	 282.33s user	  7.91s system
MPEG Audio Decoder 0.14.2 (beta) - Copyright (C) 2000-2001 Robert Leslie
     Title: sunspot
    Artist: Banco de Gaia
     Genre: Other
 00:06:59 Layer III, 128 kbps, 44100 Hz, joint stereo (LR), no CRC
error: frame 16041: lost synchronization
16041 frames decoded (0:06:59.0), -0.3 dB peak amplitude, 0 clipped samples
  412.08s real	 400.01s user	  7.55s system
\h:\w$ time madplay -m -v --output=raw:/dev/null sunspot.mp3 ; time madplay -v --output=raw:/dev/null sunspot.mp3
MPEG Audio Decoder 0.14.2 (beta) - Copyright (C) 2000-2001 Robert Leslie
     Title: sunspot
    Artist: Banco de Gaia
     Genre: Other
 00:06:59 Layer III, 128 kbps, 44100 Hz, single channel, no CRC
error: frame 16041: lost synchronization
16041 frames decoded (0:06:59.0), -0.8 dB peak amplitude, 0 clipped samples
  292.17s real	 282.48s user	  5.43s system
MPEG Audio Decoder 0.14.2 (beta) - Copyright (C) 2000-2001 Robert Leslie
     Title: sunspot
    Artist: Banco de Gaia
     Genre: Other
 00:06:59 Layer III, 128 kbps, 44100 Hz, joint stereo (LR), no CRC
error: frame 16041: lost synchronization
16041 frames decoded (0:06:59.0), -0.3 dB peak amplitude, 0 clipped samples
  412.64s real	 400.27s user	  7.42s system
\h:\w$
Script done on Thu Feb 27 15:48:08 2003

What you should look at is the 'real' column compared to the length of
the song (419 seconds).  As you can see it's fast enough to play in
mono, but not stereo.  Maybe if you had the slightly faster MobilePro
880 and some kind of USB headphones or NAS-compatible 802.11b cordless
headphones, then you could play MP3s.  I don't know about Oggs because
I couldn't get 'tremor' to build.  It has an indirect build dependency
on _Perl_ which is apparently broken on hpcmips, at least in
current-20021205.  sorry!

-- 
haven't you heard it's a battle of words                               
the poster bearer cried                                                
listen, son, said the man with the gun                                 
there's room for you inside