Echo iMac sound to both internal speakers and HDMI?

[Kadath]

I just hooked up my iMac to a receiver and speakers, the issue is that the receiver is hooked up to 4 other devices as well. I'd like to have the option of kicking sound over to the receiver about 10% of the time so the internal speakers need to keep working. I've got the screen mirrored so that I don't have to worry about that but I'd like to have sound echoed to both the HDMI and the internal speakers so I can continue to work whenever the receiver is tuned to another device. Googling has failed me, anyone have any ideas?

BTW I thought of Airfoil, but it doesn't seem to be what I need:
https://www.rogueamoeba.com/airfoil/