Use 35% opacity for faded waves in audio-volume icons and make mute waves faded

For consistency with https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/40, I've changed the opacity of faded sound waves to 35%. I also made the sound waves of the mute icon faded so that they wouldn't suddenly jump up to opaque when you lower the volume to 0%.

cc: @teams/vdg