When initializing the mixer library, one the arguments is the number of channels. One channel means mono sound and two allows stereo output. When you want to play a sound, once again channel is an argument. Channel in this case refers to the mixing channels which default to eight and are entirely separate from the audio channels. The majority of the source is found in a small number of files.
Most files define numerous global variables.
There are plenty of comments explaining the various sections so it is fairly easy to understand what is going on. Unfortunately battalion.c seems to have a rather haphazard placement of functions which makes finding a particular function more difficult. While exploring the source to Battalion, I was surprised to find the monster models hard coded in the source. My initial thought was that this may have been done to simplify animation. However, Andy Johnson informed me that the restriction was due to the hardware limitations of 1995 low-end SGIs.