The AM/FM radio will continuously play back the last station assigned from the TUNER output on the rear of the DN-300Z. The DN-300Z does not have a volume adjustment feature and you will have to turn down the volume on the connected output - whether that is a channel on a mixer or the speakers themselves.
In most cases, the DN-300Z will have the TUNER and the CD/USB/SD/AUX/BT outputs connected to a mixer of sorts and the individual channels on the mixer would need to be reduced.