If the "Input Level" value is too low, it might be difficult to hear the sound that's being input. Press the [MIC/LINE IN SETTING] button to access the GENERAL screen, and check the "Input Level" setting.
As appropriate for the equipment that's connected, also check the following points.
Type of the microphone
RD-88/RD-88 EX supports dynamic mics. Condenser mics and electret condenser mics that require phantom power are not supported.
Connecting an audio player or similar device
If you make connections using an audio cable that contains a built-in resistor, the volume will be low. Use an audio cable that does not contain a built-in resistor. Also check whether the audio player's volume might be lowered.