A silent recording is rarely a single fault. Checking the four stages — capture, encoding, recording and playback — in order is quickest.

1. Does the device support audio? Some video-only models have neither a built-in microphone nor an audio input. Those need an external microphone or a different model.
2. Is audio encoding enabled? Check the audio switch and codec in the camera's audio/video settings (typically G.711A, G.711U or AAC). Disabled audio, or a codec the recorder does not support, produces silence.
3. Was audio recorded at all? Many recorder channels default to video only and need the channel switched to audio/video mode.
4. Playback settings: client mute, low system volume, or a browser that cannot decode the codec all look like silence. Use the dedicated client for playback.

With an external microphone, always power it separately and use shielded cable; otherwise expect hiss and mains hum.