I have tried different files and formats. I have also tried al_filename_exists to verify the paths, and got 1 back.
Edit: Well, I wrote some test code to isolate all initialization to just an Allegro window and the audio subsystems and was able to load and play a sound. What this means, who knows yet... boy is this fun!
Edit 2: I seem to be able to only load 16-bit WAV files. 24-bit WAV doesn't work, but neither does OGG nor MP3. This is really puzzling because I used to be able to - seeing as I released an entire game that used tons of ogg files.
Edit 3: I found the problem. It seems that my Windows installation doesn't have the dependencies acodec uses in the path, and lazy-loading of the libvorbis DLL is being utilized when an ogg sample is loaded for the first time. My latest log file shows that libvorbis isn't being found.