I'm pretty sure that you should be able to block the PlayBackEvent like any other forward. But, it still might not change the sounds to the player firing, since it's probably done on his client as soon as he shoots, before he even lets the server know. Or something.
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I don't think that you can change an argument and then pass the forward along. You'd have to call the function again, with your own arguments, and then use FMRES_SUPERCEDE to stop the original forward from taking place.
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No longer around. Thanks your support, everyone! As always: