iMac with spotify playing music on an external sound device (Beoplay speaker). The sound device has been connected via the "Connect to a device" button located bottom-right in next to the spotify volume control in the MacOSX spotify application.
When the spotify application is playing music I can use the <previous>, <play/pause>, and <next> buttons located top-right on the apple keyboard to control music playback in spotify. That is fine.
The apple keyboard buttons <mute/unmute>, <volume down>, <volume up> however, appears to be locked to control the internal speaker and does not affect the sound playing on the external sound device.
I realize that the keyboard shortcuts <cmd+up> and <cmd+down> exists to control the volume in spotify, which will in turn control the sound level on the external sound device.
The <cmd+up> and <cmd+down> keyboard shortcuts come with a few shortcomings:
They only function when the spotify player has focus, and
They seem to have their own (incorrect) sense of the current volume level. If you have adjusted the volume with the mouse from high to low, and then press <cmd+down> the volume will go "down" from the previous high level, effectively blasting your speakers up when you really wanted to lower the volume. Not great, to put it mildly.
It would be much better usability to be able to bind the designated apple keyboard volume keys to control the spotify volume.