Audio not included in .mkv file

[Philius]

Did you read the guide?

Yes

What is the base version number of MotionPlus being used?

0.1.x

What was the install method?

Built from source code

What is base architecture?

x86/x64

What is the distro being used?

Debian

Disto version number

12

Camera/Sound type(s) being used?

v4l2 camera, Sound via PulseAudio

Describe the issue/problem and steps to reproduce

I'm using a USB camera, with integrated microphone.
The video device was /dev/video0 as it's a v4l2 device.
In camera1.conf I have v4l2_device /dev/video0.
I used pactl list short sources to get the name of the sound device.
The audio triggers from sound1.conf

snd_device alsa_input.usb-HD_WEB_CAMERA_HD_WEB_CAMERA_SN0001-02.mono-fallback
snd_params source=pulse
snd_show on
snd_alerts alert_id=01,volume_level=01,volume_count=10,freq_low=100,freq_high=3399,alert_nm=alert

Show up in the log.
I've tested the device with
ffmpeg -f pulse -i 'alsa_input.usb-HD_WEB_CAMERA_HD_WEB_CAMERA_SN0001-02.mono-fallback' -ac 1 recording.m4a
and it works, as verified with ffplay recording.m4a.
But the recorded .mkv file is missing audio.

Relevant MotionPlus log output (at log_level 8)

Mar 20 17:50:05 [INF][ALL][01:sl01:MyMic1] snd_check_alerts: Sound Alert 1-alert : level 1 count 2015 max vol 81

[Mr-Dave]

v4l2 devices are not those that are supported for recording audio. The docs say certain cameras and those are currently rtsp cameras with passthrough specified.

The sound device / alert is for frequency detection.

[Philius]

I added the sound alert to show that the device is being used correctly.

[Philius]

The audio input is a pulseaudio device.
snd_params source=pulse
Are those supported?

Note that my goal is to use the raspberry pi camera with a USB mic, testing on a PC is just easier.
Is the raspberry pi camera + USB mic combination supported?