Well, I have done more experimenting with this. mpg123 really, um, stinks. Ran
into a small problem with the input buffer not being big enough (it defaults to
0, so I bumped it up) but the thing likes to chop off the last few seconds of a
song. Plus, every time I do anything on the machine the player goes into
stutter mode. 

So, I found FreeAmp. Awesome package. I dumped XMMS as soon as I saw it. XMMS
will not read from a tape at all that I can see. But FreeAmp does - and it does
not exhibit any of the stuttering/chopping songs off like mpg123. Just do a
freeamp /dev/tape and it starts running. 

Anyway, if anyone is looking to play mp3s off a tape drive, those are my
experiences.