The request is pretty straight-forward. I’d like to request the ability to control each individual player’s volume from the client. For example:
// Sets the volume that this client hears the specified player at.
//
// @param player - The player to have their volume changed.
// @param volume - The amount, between 0.0 and 1.0, to set the volume to.
OVERRIDE_PLAYER_VOLUME(Player player, float volume);
Good idea.
I believe it will be implemented when the voice chat will work fine with the onesync API maybe.
Like this feature 
Mumble VOIP hook "canHearPlayer"
You don’t really need this since mumble is implemented in the core of fivem.
Mumble has 5.1/7.1 capabilities and has positional audio.
This system has been implemented in some games by 3rd party, where this was a great success.
So all i can say wait a little while for the devs to finalize mumble implementation.
OR maybe look into it yourself and throw an helping hand into onesync development on github? 
This would be beneficial for simulating multiple voice channels, which GTA supports the ability to hear multiple players regardless of voice channel, however mumble does not currently support this (see issue #3319).
As for “not needing this”, I don’t see why it wouldn’t be beneficial to give resources the ability to control this, considering that other natives could be used to control mumble (i.e. NETWORK_SET_TALKER_PROXIMITY).
Also, while yes, it is an open sourced project, that doesn’t necessarily mean that every pull request or feature request will be adopted. Also if every feature request is met with the response of “do it yourself”, there’s no point of a feature request.
We’re intending on implementing this based on dynamic voice targets, though I am not sure if that would cover all the likely use cases.
Anyway, a volume multiplier seems simple - though it’d still mean a lot of data might get routed for no reason when a player is muted. 