mouse scroll wheel can't set volume with .mp3

When running a video, .mp4 or .flv or .mkv, the volume can be adjusted by clicking on the window and using the scroll wheel.

With an .mp3 running, clicking in the main window or on the playlist bar on the left, I can't adjust the volume with the scroll wheel. If clicking on the control bar at the bottom, between the control buttons and the volume icon, the scroll wheel can adjust the volume.

The expected behavior would to allow clicking in the window to allow the volume to be adjusted.