If you record sound from a microphone connected to a USB audio interface, it might be that you only get an input signal on the left stereo channel. This is because the second input on the audio interface goes into the right channel, a behaviour that seems to be common for audio interfaces with two input channels. For recording audio, this is not a problem since you can reassign this in any audio editor. Video call software might also work, since it probably downsamples the input to mono, so you could be fine. Nevertheless, it is annoying and can lead to noise from the second, unconnected input to be picked up. Here, I’m telling you how to extract the correct inputs cleanly in Pulseaudio on Linux so that it can be used by any software.

But first, if it’s just for OBS, you might be happy with clicking the settings menu in the Audio Mixer, going to “Advanced Audio Properties”, and setting your mic to “Mono” (disabling everything except Track 1 on the right).

Now back to Pulseaudio. It seems that there are patches for some devices that will be included in future versions. Then you can presumably choose the left and right channels as mono inputs via device profiles in pavucontrol. For now—or if your device is not supported—you can make a new “virtual source” from a hardware audio source. For this, you first need to find the name of your device. You can use pacmd list-sources in the command line. For the Behringer U-Phoria UMC22 (and probably others with the same chip) it is alsa_input.usb-Burr-Brown_from_TI_USB_Audio_CODEC-00.analog-stereo. Now, create the file ~/.config/pulse/default.pa with the following contents:

# Ensure the system configuration is loaded first.
.include /etc/pulse/default.pa

# Behringer U-Phoria UMC22 - mono inputs
load-module module-remap-source source_name=UMC22-Left-to-Mono  source_properties="device.description='Behringer UMC22 - Microphone'" master=alsa_input.usb-Burr-Brown_from_TI_USB_Audio_CODEC-00.analog-stereo master_channel_map=left  channel_map=mono
load-module module-remap-source source_name=UMC22-Right-to-Mono source_properties="device.description='Behringer UMC22 - Instrument'" master=alsa_input.usb-Burr-Brown_from_TI_USB_Audio_CODEC-00.analog-stereo master_channel_map=right channel_map=mono

Change the master=… setting to the correct device you found earlier. Restart Pulseaudio with systemctl --user restart pulseaudio.service.

Notes: The user most likely needs to be a member of the audio group. This whole setup only works if the device is plugged in when Pulseaudio starts. If plugging in later or in case of problems, restart with systemctl --user restart pulseaudio.service.

Now you have two more inputs called “Behringer UMC22 - Microphone” and “Behringer UMC22 - Instrument” that you can chose in any software that supports Pulseaudio. Feel free to change the string after device.description to change the name.