Change volume of playback ports by default

This seems to be more correct than changing volume of the whole sink.
The default volume of the sink is 1 (100) and alsamixer also changes
volume of the ports.

pw-lib.el

@@ -150,6 +150,13 @@ version, call `pw-lib-refresh' first."
   "Return a PipeWire object that is the current default audio sink."
   (pw-lib-get-object (cdr (assoc "default.audio.sink" (pw-lib-default-nodes)))))
 
+(defun pw-lib-default-playback-ports ()
+  "Return list of PipeWire objects that are default playback ports."
+  (cl-remove-if-not #'(lambda (o)
+                        (if-let ((name (pw-lib-object-value o "port.name")))
+                            (string-match "^playback" name)))
+                    (pw-lib-children (pw-lib-object-id (pw-lib-default-audio-sink)) "Port")))
+
 (defun pw-lib--volume-% (volume)
   (when volume
     (round (* 100 volume))))