Are you able to enter OpenFirmware-console useing the Radeon-card?
If yes, you may boot MorphOS manually:

>boot cd:,\mac_ppc64\boot.img

Then you can see what happens: Is kernel loading? Does Morphos-System load?

If you cannot enter Open Firmware, your Radeon (PC-flashed card?) is not recognized by OF and if MOS won't boot, you cannot see what's the reason for.
Very strange that there is no boot-chime with radeon-card. If this card fits to physical specs of slot (electricaly) then there should be a chime, even if card is not initialized by OF.