I have a question about the sound buffer size in FastDOOM. I see a 16x buffer (NumberOfBuffers) instead of a standard double buffer used. I'm wondering what led to that decision? I've looked into the SB mixing code and its not clear to me that it has any kind of advantage. Is it maybe non-soundblaster related or am I missing something?

That value is the default for the Apogee Sound System. I'm not exactly sure why they use 16 buffers for sound mixing. Maybe it's set that high for sound cards that require multiple samples to be prepared in advance before being sent to the device. For example, the Disney Sound Source could receive up to 14 sound packets.