Can I use my Mac's speakers and AirPlay / Bluetooth at the same time? I have an iMac at my home office that I'm often playing music on from the output speakers. I also have an AirPort Express with a speaker connectable to via AirPlay / Bluetooth. I was wondering, is there any way to play my music from the standard output on my iMac, and at the same time, play it from my AirPort Express using AirPlay or Bluetooth?

Reference:

iMac 27" Late 2013 running OS X 10.11.3

AirPort Express Dual-band 802.11n
Try using the midi setup application included in OS X it can let you group sound outputs together. You just need to create a new multi-output group with the + button.