OK, this works for me. Even sounds OK, given my PC speakers, and
SoundBlaster clone card...

These assume that your Unix/Linux box has ffmpeg and lame
installed and on the path visible to slimserver....

Create a file in /usr/local/slimserver called slimserver-convert.conf

When the slimserver starts, it will read this file and over ride
anything in the convert.conf
(the advantage is that while the convert.conf may be over written by
re-installations, the slimserver-convert.conf should be safe forever.)

Include the following lines in the slimserver-convert.conf

wma wav squeezebox *
[ffmpeg] -v 0 -i $FILE$ -f wav -

wma mp3 * *
[ffmpeg] -v 0 -i $FILE$ -f wav - | [lame] -x -m s -r -S -q 5 -s
44 - -

The first sends WAV files to all squeezeboxes, using ffmpeg
the switches say "no video, take input from a file and output a wav pcm file"

The second sends MP3 files to anything that isn't a squeezebox
The ffmpeg switchers are the same, then we pipe it thru lame which
converts it to MP3 and pipes the output wherever the slimserver says.

Switches are "correct for bad endianess, -mode stereo, raw PCM input
silent quality 5 sample rate 44 kHz"
you can use whatever quality number your server can handle.

There is probably a switch to ffmpeg that would negate the need for the -x
in lame,
but I can't see it. And this works.