This post explains how to attach AX.25 to a soundcard. This was difficult for me to find out how to do on the web, so hopefully this helps someone else. As of 11/7/2021, this was tested in Kali Linux, so probably will work in Debian and Ubuntu, but this isn’t verified.
Install the prerequisites.
> sudo apt install ax25-apps ax25-tools direwolf
Execute the following command. This will attach your soundcard to a /dev/tty interface.
> direwolf -p
Verify that your /etc/ax25/axports file has an entry for port 1. “man axports” will tell you more if needed.
In a separate terminal, execute
> sudo kissattach /dev/pts/1 1 <IP address of interface>
This attaches AX.25 to your soundcard. Pinging an IP address in the interface’s subnet should produce an audio output. Executing the command below will help you verify that the interface is up and the subnet is correct.
> ip addr show
You might see alerts that the input audio is too loud. You can use the command “pavucontrol” to manipulate the input audio to get it to a good level.