softphone impl. with ability to 'play-in mp3 files'

[Richard P. Williamson]

Hi,

I'm trying to find a specific software package for a trade-show
demo.

We need to be able to play an MP3 file (actually, a 'book on tape')
across a SIP VoIP link as part of the demo (so we don't have to
embarrass ourselves by holding a one-sided conversation), but I
can't figure out which terms to google to find what I'm looking
for.  

Anyone out there know of a product/pd/sourceforge/package that
will do that (command line, access an mp3 file and play it across
a SIP initiated VoIP link)?

Thanks ia,
rip