The Astro A20 headphones are NOT being registered as an output device on Ubuntu 20.04, even though the built-in mic is registered correctly as an input device.
Append the function below to your ~/.bashrc and then you can simply type in a20 in the terminal whenever you need to connect your A20 headset.
It checks to make sure the headset is plugged in and uses a regex to automatically detect the card number, which often changes. The device number on the other hand seems to be always "1" (at least for me).
If you want to connect the headset at startup, simply copy the function body (what's inside the curly brackets) to ~/.profile.
# Astro A20 headphones
