Use command admin_voice <1/0> to set speak/listen all
Cvars:
av_reset_on_spawn "1" // Reset admin_voice on spawn
Flag -> Admin_Chat If you want change this, you can set flag on file addons/sourcemod/configs/admin_overrides.cfg Add:
"admin_voice" "<flag>"
and change to your flag https://wiki.alliedmods.net/Adding_Admins_(SourceMod)