Hit the FM stereo switch, set it to mono. Do you get any sound now?
When set to stereo the circuit is designed to mute any audio if the
quality and strength of the signal being received is'nt stereo worthy
( better antenna/ orientation required). Your RDS is working , else
i'd suspect FM detector transformer.
Loading 2 pairs of speakers on would'nt cause this problem , as the
rest have stated.
Jango.