I explored the innards of the app, and discovered it does three things:
1) Uses the Macintosh-built-in /usr/bin/say program to create an AIFF file from the text;
2) Uses a copy of LAME included in the download to convert the AIFF to MP3; and
3) Deletes the AIFF file.

The problem appears to be that the Mac program /usr/bin/say is very finicky about what it will 'speak'. The whole CR-LF vs. LF thing seems to be enough for it to barf; other text like long hyphens instead of double-dashes (— vs. --), and 'curly quotes' instead of plain-vanilla-ascii quotes also may play a role.

It should be tough to add a sed script to the front end to clean out everything but plain ASCII, but I'm already over the time limit I'd allocated for this :-)