The audio module requires audio files in 4-bit ADPCM (.ad4) file format. There is a converter utility on the Sparkfun product page. I found that not all WAV files that I converted were able to play, so I just tried a lot until I had a big enough number of files to make it not repeat itself too quickly.
On the audio board when the 'next' signal goes active, the current clip is stopped and the next one is played. This did not work very well with the motion sensor, which outputs a lot of pulses when active. To finish every sound clip and wait for the next pulse, the motion signal was or'ed together with the busy signal from the sound module and connected to the 'next clip' input. This guaranteed to finish playing the current track before the next track is played.
I added a video that demonstrates all the scary sound clips: