Since Brekeke PBX is a PBX application, it is aware of audio content.. so you can use only codecs which Brekeke PBX supports.
But.. if the PBX doesn't relay RTP, you can use the PBX with any codecs as voipwell said.
FYI: if you use Brekeke SIP Server instead of Brekeke PBX, you can use any codecs even if the SIP server relays RTP. This is because Brekeke SIP Server is a SIP proxy and it doesn't care about audio content.