If I remember right, the easiest and 'only' way is to modify
lines 626-630 in cl_voice.lua of the gamemode. This isn't recommended, as it will probably need to be modified again after a Garry's Mod update, but I don't think there's currently any other way.
Should look something like:
if ply:IsAdmin() or ply:IsDetective() then
return GetGlobalFloat("ttt_voice_drain_admin", 0) -- Default 0.05
elseif ply:IsUserGroup( "vip" ) then
return 0.1
else
return GetGlobalFloat("ttt_voice_drain_normal", 0) -- Default: 0.2
end
0.1 is just an example, since it's half of the drain rate of normal players, thus giving twice the battery for "vip". Change this how you would like it to be.