There is no built-in mic for RLC-423/RLC-423S/RLC-423WS, so if you'd like to get audio, you need to install an external microphone for RLC-423/RLC-423S/RLC-423WS.

You may follow the article below to install the microphone for RLC-423 series cameras.

How to Choose Microphone for RLC-423/RLC-423S/RLC-423WS

How to Install the Microphone for RLC-423/RLC-423S/RLC-423WS

And you could follow the article below to enable audio on RLC-423/RLC-423S/RLC-423WS.

How to enable audio for recording and live viewing