Okay, so I did it.  B-)

I'm working on a modification to mpg123 to use fixed-point arithmetic.
Almost all of the code touched in the layer 3 path (the antialiasing
and hybrid filters, and the subband synthesis) are converted, and it's
running about 2.5x faster than realtime on my Shark, decoding 192Kb
`CD-quality' audio.

There are still a few things to do (fixing the layer 1 and 2 paths and
the equalization filter, and converting the dequantization portion of
the layer 3 code), but it's looking good so far!