There is no built-in mic for RLC-423 so 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 RCA port.
About the microphone, please kindly refer to Tiny Spy Microphone. Also, you need an RCA Male to RCA Male Adapter Connector with it.
All the above is only for reference and if your microphone supports the RCA socket. It also can work.