Pulseaudio: remap source when usb audio interface is connected

I'm not sure if this would help on boot, but for disconnecting & reconnecting you can try to use udev. I do this for my MOTU M2, but presumably it could be tweaked for your device. First find out what udev knows about your device. Run udevadm monitor in a terminal and connect your device. You should see a line like:

KERNEL[425578.748455] add      /devices/pci0000:00/0000:00:1d.0/usb2/2-1/2-1.6 (usb)

(followed by many others)

Copy the /devices/ path and run:

$ udevadm info -ap /devices/pci0000:00/0000:00:1d.0/usb2/2-1/2-1.6
  looking at device '/devices/pci0000:00/0000:00:1d.0/usb2/2-1/2-1.6':
    ...
    ATTR{product}=="M2"
    ATTR{manufacturer}=="MOTU"
    ...

Now create a .rules file in /etc/udev/rules.d/, (you could call yours 99-roland.rules or something).

--- /etc/udev/rules.d/99-motu.rules ---

ACTION=="add", ATTR{manufacturer}=="MOTU", ATTR{product}=="M2", RUN+="/home/marf/self/bin/motu-udev.sh"

Obviously you'll need to change the ATTR parameters to match what you found out with udevadm info and the script location to match where you want to put the next script.

Tell udev to load the new rules:

sudo udevadm control --reload-rules

In the script pointed to by the RUN parameter, you can run pactl commands to setup pulseaudio how you want for when your device is connected. (If you also need to do things on disconnect, you could use ACTION="remove" in the udev config and point to a different script if desired.)

For my device, unfortunately it doesn't show up in pulseaudio until after udev finishes, so I have to spawn a separate script and wait for it to show up. I'm not sure if this is specific to my device, or if it's just because pulseaudio is also using udev to add the device. Also, the scripts from udev run as root, but pulseaudio is usually running as your unprivileged user account. The first few lines are to figure out what that account is (this part I found in the Arch wiki). Here are my scripts:

--- motu-udev.sh ---

#! /bin/bash
export PATH=/usr/bin:/bin
USER_NAME=$(who | awk -v vt=tty$(fgconsole) '$0 ~ vt {print $1}')
USER_ID=$(id -u "$USER_NAME")
PULSE_SERVER="unix:/run/user/"$USER_ID"/pulse/native"
/home/marf/self/bin/motu-pulse.sh "$USER_NAME" "$PULSE_SERVER" & disown

--- motu-pulse.sh ---

#! /bin/sh
USER_NAME="$1"
PULSE_SERVER="$2"
sleep 2

if ! sudo -u "$USER_NAME" pactl --server "$PULSE_SERVER" list modules | grep master=alsa_input.usb-MOTU > /dev/null; then
        sudo -u "$USER_NAME" pactl --server "$PULSE_SERVER" load-module module-remap-source master=alsa_input.usb-MOTU_M2_M2MT078C8F-00.analog-stereo master_channel_map=front-left channel_map=mono source_name=MXL-M2 source_properties=device.description=MXL-M2
fi
sudo -u "$USER_NAME" pactl --server "$PULSE_SERVER" set-default-sink alsa_output.usb-MOTU_M2_M2MT078C8F-00.analog-stereo

So udev triggers on the device's USB connect, and runs motu-udev.sh. Then motu-udev.sh spawns motu-pulse.sh and disowns it so udev can finish processing (and actually create my device in pulseaudio). The motu-pulse.sh script sleeps for a bit to wait for that to finish, and then it creates the channel mapping if necessary, and sets the device as the default output. (If you only want to use it as an input device, you can leave out the last line)

Some resources I found helpful were the Arch wiki and this tutorial:

https://wiki.archlinux.org/title/PulseAudio/Examples
https://linuxconfig.org/tutorial-on-how-to-write-basic-udev-rules-in-linux

Let me know if you find any useful changes, or if there's a way to do this purely in Pulseaudio without udev!

Pulseaudio: remap source when usb audio interface is connected

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