pipewire: Add configuration for UMC202HD

This has some problems (as explained in the comment), but this at least
makes it work for now.

modules/pipewire.nix

@@ -23,4 +23,47 @@ lib.mkIf config.sbruder.gui.enable {
 
   hardware.bluetooth.enable = lib.mkDefault bluetoothSupport;
   services.blueman.enable = lib.mkDefault bluetoothSupport;
+
+  environment.etc."pipewire/pipewire.conf.d/10-behringer-umc202hd.conf".text = ''
+    # This is heavily based on the example from the PipeWire wiki:
+    # https://gitlab.freedesktop.org/pipewire/pipewire/-/wikis/Virtual-Devices#behringer-umc404hd-microphoneguitar-virtual-sources
+    # It has the problem that the virtual source persists,
+    # even when the physical source is disconnected.
+    context.modules = [
+        {   name = libpipewire-module-loopback
+            args = {
+                node.description = "UMC202HD Left"
+                capture.props = {
+                    node.name = "capture.UMC202HD_Left"
+                    audio.position = [ FL ]
+                    stream.dont-remix = true
+                    target.object = "alsa_input.usb-BEHRINGER_UMC202HD_192k_12345678-00.analog-stereo"
+                    node.passive = true
+                }
+                playback.props = {
+                    node.name = "UMC202HD_Left"
+                    media.class = "Audio/Source"
+                    audio.position = [ MONO ]
+                }
+            }
+        }
+        {   name = libpipewire-module-loopback
+            args = {
+                node.description = "UMC202HD Right"
+                capture.props = {
+                    node.name = "capture.UMC202HD_Right"
+                    audio.position = [ FR ]
+                    stream.dont-remix = true
+                    target.object = "alsa_input.usb-BEHRINGER_UMC202HD_192k_12345678-00.analog-stereo"
+                    node.passive = true
+                }
+                playback.props = {
+                    node.name = "UMC202HD_Right"
+                    media.class = "Audio/Source"
+                    audio.position = [ MONO ]
+                }
+            }
+        }
+    ]
+  '';
 }