Thanks for the help!
I'm working on an UNO, and once I uncommented those lines, it (almost) worked. 
For anyone else who is having trouble - in order for my audio files to play, I had to put ".WAV" after the file name in the code (not in the actual file name on the SD card). For example, the basic example code uses audio.play("music") - change that to audio.play("music.WAV"), but make sure that the actual file name on your SD card is "music".
I also had to make sure the library was updated before it worked.
You may want to fiddle around with the sound quality settings specific to your pieces of audio, and find out which sounds best.
Finally, if the audio isn't sounding quite right or is distorted, you could try calling the audio.setVolume() function... 1 is lowest volume and 7 is highest; I found that 6 and 7 were too high for the audio samples I worked with.