Mac Mini startup sound always plays on internal speaker My 2012 Mac Mini (Mavericks) is plugged into a USB audio interface which is selected as my sound in and sound out device. All works great, except for the startup sound which still comes out of the internal speaker. Is there a fix for this?
Seemingly this is a feature and you can't change it. Other Mac have the same problem. You juste can mute the sound of the internal speaker during the boot.

https://superuser.com/questions/770364/why-does-my-macbook-play-the-startup-sound-through-the-speakers-rather-than-thro