how do you do this
i made a flash grenade which can deafen players but it's very janky
Probably the same way you did in BCC. You can package out sounds that are played directly to a client, but sounds that are played with serverPlay3D have to be culled out by just beating the stuff out of their client with null sounds. I haven't tested this with more than one person so there's no guarantee it doesn't cause performance issues/actually works.
The proper way to do this would be to lower volume on the client-side which can only be done in singleplayer, like how I made vertical/horizontal recoil work in singleplayer. Until methods like this are exposed to the server there's nothing I can do about it, maybe the guy who joined my server to scream at me about how making a DLL to break my addon was easier than typing a single slash command can do it for us.