Getting sound to split between computer and external monitor (TV hooked up with HDMI to thunderbolt) from iMac but NOT simulatneously

I connected my TV to the iMac last night. When I drag a program up to the TV the sound switches from my computer speakers to my TV speakers; I thought this was a great feature. However, when I wake up the computer the speakers are only working from the iMac. I know how to switch the sound back to the TV and how to play them at the same time, but I just can't get back to that smart audio feature (where the sound comes from the speakers within the display showing the program) that I appreciated so much the first time. I have scoured the web looking for answers and even called Apple support. How can I re-enable this feature?