I've downloaded the mod and had a short look at the weapon shot files.
I randomly checked 3 of them and the sound file were all 4-5s long (in Vanilla it's usually just a fraction of a second).
The maximum amount of sound channels supported by modern hardware is usually 256. Let's take an example of a MG (5s is the duration of the shot sound file) that shoots 5s long at 10 shots/s, you'll have 50 sounds stacked after 5 seconds. Would 6 soldiers do that at the same time, it would exceed the maximum 256 sound playing instances supported.
here is one of those files:
as you can see, there are about 3.6s which can easily be cut out of the WAV file which the mod author shoud do to avoid such issues