Ok here goes my attempt at clearing this up:
The RTR servers, and probably a lot of other servers are using the 716 Sound mutator. There is a thread about the mutator here:
http://www.redorchestragame.com/forum/showthread.php?t=15258
The server installs this mutator which is then downloaded by the clients when they join the server. This particular mutator modifies the weapon sounds only but there are also mutators for HUD modifications, weapon code, etc. Everyone who plays on the server will have this mutator applied so you will all hear the same sounds.
Mutators work on a per-server basis so if you join a server without this mutator the sounds should go back to normal. You can check which mutators (and other server info) are in effect by looking at the lower left box in the main server browser after highlighting the server you want to view.
Personally, I believe the stock RO sounds are very well done and this mutator should not be used in its current state. This mutator is using Copyrighted Call of Duty sounds and also does not properly handle the distance changes between weapon sounds very well in my opinion. If you want the server to revert back to the original RO sounds your best bet is to contact the people who run the server. In the case of the RTR servers that's RightToRebel.net
Background sounds are a different story and are up to the mapper, not the server or sound mutators.
Hopefully this clears things up a little.