Lots of people, me and my friends included, enjoy using the DragonFyre and DynaSound mods.
They greatly enhance the sound quality of ARMA 3, and make the game sound a lot cooler.
However, using either of these mods causes the wasteland server to kick you instantly when trying to join.
Allowing users to use either mod is as simple as adding the mod's server key into your server files and will not affect players without the mod.
P.S. If there can only be one or the other, DynaSound is a whole lot cooler