There is no built-in mic for RLC-423. If you'd like to pick up audio, you need to connect an external mic to the "Audio-in" port of RLC-423. The port is an RCA port.
About the microphone, please kindly refer to Tiny Spy Microphone. You also need an RCA Male to RCA Male Adapter Connector with it.
All the above is only for reference. If your microphone supports the RCA port, it will work.