No Sound on a Motorola DCP-501

You don't say what else works. Have you tried it with FM to eliminate external source causes? Does your TV get audio passed through to it? That would establish that there is program material to be amplified. Headphones work?

The following is from the User Guide -

Check to make sure that each speaker has been properly connected to the
speaker output jacks on the rear panel of the DCP501.
�� Check that the speakers are working by utilizing the Test Tone function as
described in the “Operation” section.
�� Verify that the DCP501 has not been placed in MUTE mode.
�� Check that the volume control on the DCP501 has been adjusted to an
appropriate listening volume.
�� Verify that the DCP501 is set to the correct source (CATV, VCR, DVD, Tape,
etc).
�� If the front panel displays O V E R L O A D , one of two things has happened:
1 If the DCP501 detects the speakers have a short circuit, it shuts down
the amplifiers. Switch the DCP501 off, remove the short circuit, and turn
the DCP501 back on. The DCP501 should now work as normal. If
O V E R L O A D is still displayed, a short circuit condition remains.
2 If the maximum amplifier output power is exceeded, the DCP501 shuts
down the amplifiers. Turn down the volume a little and switch the
DCP501 off and then switch the DCP501 back on and the DCP501
should now work as normal. If O V E R L O A D is still displayed, turn the
volume down some more.

The Speaker Setup menu has a Speaker Reset (to default settings) option.