Currently: The shortcut ⌘ + volume up/down has no effect if the playback in the targeted music app(s) is paused or off.
Proposal: Make the app volume change also possible if no audio is playing.
Use case: Setting the intended volume BEFORE starting playback, because you want to prevent the start of your playback being too loud (irritating/ear-damaging) or a too silent (may go unnoticed).