To enable audio-only recording you have to first send a video signal to the Video Channel so that the audio signal can be sent to it. Once configured you can remove the video signal for confirmation please check the audio on the encoder or Valerus.
How to check via the encoder is shown below.
As a side note, Louroe makes powered microphones that work with the VLR
They have a pair of wires for the audio and another pair to supply power to the mic.
https://www.louroe.com/wp-content/uploads/2019/12/amic_inst_12_14.pdf
Assuming the Mic is providing line-level audio, confirm audio can be heard at the Encoder
https://www.vicon-security.com/wp-content/uploads/2017/10/ENC-H264-16_QuickGuide.pdf
As you know the audio inputs are assigned to specific cameras.
The guide details live audio and video on its local monitor. Please confirm it works at the VLR
Inputs for a mic on connectors "1 "
; 4 audio in RCA; audio out: 1 RCA;
audio output to a powered speaker
audio out: 1 RCA to a powered speaker