I got my audio to work...
I followed the guide at this site to test my audio: https://sites.google.com/site/semilleroadt/home/raspberry-pi
I also learned that Midori doesn't allow file downloads. (Or at least it doesn't work for mp3's) Basically, my mp3's were invalid because I unsuccessfully tried downloading them with Midori. The filename shows up but upon closer inspection, its filesize was 0 bytes! (I only came to this realization after going through the audio test above and performing a command line download of their test audio file.)
I used a different browser to download mp3's and now the program works with sound.
One minor potential issue with the code though...
I noticed that if you hold the button a little longer than your average button press, then the program seems to try to play multiple instances of the same sound clip simultaneously. The speed at which this appears to be controlled might be the "sleep" command? If set at 0.1, I assume it will loop through the polling loop quite fast and perhaps overwhelm the cpu?
Well, I seem to have locked up the Pi because I held the button down for a few seconds. In the amount of time it has taken me to type up this response, the Pi has not recovered, so it looks like it's time to yank the power.
I've got a lot to learn! Sigh.