public void OnClientPostAdminCheck(int client)
if (!CheckCommandAccess(client, "allow_user_to_talk", ADMFLAG_ROOT))
Very simple code, mutes all that doesn't hold the "allow_user_to_talk" override.
Or if you got a forum running for your community, you can require them to register (and link Steam account to their profile):
-> [ANY] Registered User Voice Enabler
(Edit: ... also fixed your title)