Do you have external speakers connected or trying with internal speaker? I had issues with similar PowerMac earlier, but haven't tried with the latest version. I got no sound from internal speaker if I didn't have external speakers with 3,5mm plug connected.. and then I got audio from both.
PCI audio card should be the final fix if nothing else helps...
"MorphOS/Devs/Audiomodes/I2s Make behaviour of HeadPhone mute/detect GPIOs manually adjustable via env:applei2s.audio INVHPMUTEGPIO and INVHPDETECTGPIO switches, required for some "PowerMac3,x" models to unmute audio" http://www.morphos-team.net/releasenotes/3.8
Sorry to necro-thread. But I'm having a similar issue that isn't solved by the shell command. Speakers only unmute if I have headphones plugged in. Using a PowerBook G4 running 3.11 if that makes a difference.