Raspberry Pi 5: alsamixer displaying different controls on boot

Question

I have a Raspberry Pi 5 4Gb. I am trying to run a Python script on boot which will allow me to control the volume of my default amixer device (Master) through a slider switch. The script works perfectly fine when I run it manually.

However, I am unable to access the "Master" device via amixer when running the script on boot.

• When I run the amixer controls command via SSH while logged into user pi (or running the Python script), I get this output:

  numid=4,iface=MIXER,name='Master Playback Switch'
  numid=3,iface=MIXER,name='Master Playback Volume'
  numid=2,iface=MIXER,name='Capture Switch'
  numid=1,iface=MIXER,name='Capture Volume'
  

• However, when I run the same script on reboot, I get this output instead:

  numid=1,iface=CARD,name='HDMI Jack'
  numid=5,iface=PCM,name='ELD'
  numid=4,iface=PCM,name='IEC958 Playback Default'
  numid=3,iface=PCM,name='IEC958 Playback Mask'
  numid=2,iface=PCM,name='Playback Channel Map'
  

  Note that I get this same second output if I run the command sudo amixer controls.

• I've tried running the script via crontab -e and I've also tried putting this line into my rc.local file:

  su -u pi -c "python /home/pi/scripts/volume.py > /home/pi/scripts/volume.log 2>&1" &
  

  However, both of these result in the same second output.

I believe getting the first output has something to do with running the script under the correct user or setting my environment variables correctly, but I have no idea how to do it.

Would someone be able to help me here?