[popcornmix]

However , my question is this : what would be the best approach to output the audio from music using the USB DAC and the movies using HDMI ? Going to settings every time and changing from HDMI to USB audio is a bit inconvenient. Maybe dual boot a different openelec instance ?

While it's not officially supported, if you set audio output device to USB, and enable "omxplayer" and disable "MMAL" in video acceleration settings, then
playing music uses paplayer, which will use the ALSA device. Video will be forced to use omxplayer which doesn't support ALSA, and so falls back to HDMI.