If you're using the default character sound script, I've written a network traffic-optimized version.
Sound.rbxmx (11.0 KB)
(To use, place the "Sound" script within "StarterPlayer/StarterCharacterScripts")
This problem came from @alexnewtron's 100-player meepcity servers, they were having network bandwidth problems with the "new sound code" enabled.
The old default sound code sent a bunch of unnecessary replication traffic. This new code, in the "worst case scenario" (a player repeatedly jumping up an upwards slope) should be up to 6x less network traffic, while behaving almost exactly the same to to player.
Anyway, try it out and let me know if you've got any comments.
This will be the new default within a few weeks.