Running a Thermos-1.7.10-1614-57 server jar but highly doubt that's related.
Grab a minigun from the Modern Warfare Content Pack (again assuming this affects other weapons, just being as specific as possible) and shoot some glass. The high-pitched dink sound effect can be heard at equal volume across the entire map.

So weapon sounds are heard at equal volume to every player, regardless of distance, specifically for glass breaking?

The sounds of the guns themselves are fine, it's just the high-pitched dink effect whenever a bullet hits glass that gets played globally. If you're having trouble reproducing it I can strip down my mod list a bit and try to come up with a more controlled test.