When using the RD-500 with an external sound module, you can toggle between the two sound sources by setting the RD-500 LOCAL switch to [on1], and using the UPPER and LOWER TX buttons to transmit.
To set the RD-500 to LOCAL "on1":
- Press the FUNCTION button so it is lit.
- Press the LOCAL button.
- Use the DEC and INC buttons to select "on1" in the window.
(The manual says that the RD-500 defaults to LOCAL "on1" when first turned on, however this is incorrect.)