This would not be a global sound that everyone else in the server could hear -- this would be a sound that would take priority over the default music locally, so only you would be able to hear it. Everyone would be able to do this regardless of ownership. If I run a HM, maybe I want to listen to "trololololo" and "BOOM HEADSHOT!" every time I get a kill, while I may prefer a klaxon siren if I'm not running one and just want to be alerted when one is nearby...the sky is the limit.
The upsides to having localized custom sounds is no one has to download the files while connecting to the server which may also mean no copyright issues to deal with, and it also means nobody can grief with it (you are listening to your own audio files). The downsides are that everyone's HM will be playing your choice of audio in-game and...well...you cant grief with it
