cat ~/.asoundrc

and the output of these two commands:

whereis arateconf

dpkg -l | grep alsa-config-utils

You were supposed to compile and install arateconf (along with other things), and use arateconf (in interactive mode) to generate ~/.asoundrc

The sound icon "beside the clock" is not a "mixer". It is a sound applet.
The ALSA software mixer is to be configured with the help of arateconf

A mixer is supposed to mix something, as the name suggests.
OSS4 has vmix. It is enabled by default and works out of the box.
ALSA has dmix. It is not enabled by default. It has to be configured by the user.

Open: _https://dev1galaxy.org/viewtopic.php?id=7142

Scroll to the blue line:

How to compile Petrov's fftrate ALSA plugin on Devuan 5 Daedalus

Install "build-essential", etc.

sudo apt update
sudo apt-get install build-essential git debhelper
sudo apt install libasound2-dev libasound2-plugins swh-plugins 

and so on.

The user manual is here:
_https://dev1galaxy.org/viewtopic.php?id=6644

The updated instruction for compilation is here:
_https://dev1galaxy.org/viewtopic.php?id=7142

rms@ASUS:~$ inxi -A
Audio:
  Device-1: Intel Tiger Lake-LP Smart Sound Audio driver: snd_hda_intel
  Device-2: DJI MIC MINI type: USB driver: hid-generic,snd-usb-audio,usbhid
  Device-3: Logitech C920 PRO HD Webcam type: USB
    driver: snd-usb-audio,uvcvideo
  API: ALSA v: k6.1.0-38-amd64 status: kernel-api

fuser -av $(find /dev/snd -type c 2>/dev/null)

inxi -A

You may need to install inxi

sudo apt install inxi

I would edit /etc/asound.conf defining a mono 'type dsnoop' device that corresponds to the Mic.
(use 'arecord -l' to show you the CARD names ALSA is using)

pcm.dji_mic {
        type dsnoop
        ipc_key 34848
        ipc_key_add_uid true
        ipc_perm 0666
        slave.pcm {
            type hw
            card "DJITHINGY"
            device 0  # mic
            channels 1
        }
    }

I would the wrap that device definition with another of 'type route' with two channels, which maps the
single mic channel onto both channels.

pcm.stereo_dji_mic {
    type route
    slave.pcm  dji_mic
    ttable {
      0.0 1.0 # left to left at full volume
      0.1 1.0  # left to right at full volume
    }
    channels 2
}

I would then wrap the stereo mic with type plug to manage format conversions

pcm.source {
   type plug
   slave.pcm stereo_dji_mic
}

Once configured run 'arecord -L'
if /etc/asound.conf is wrong or has syntax errors etc that command will barf with an error message you can google

The above is off the top of my head but should enable you to search for what you need.