Q: sound output keeps switching back to internal speakers after reboot

I have my Mac Mini, running 10.11.4, connected to my Sony Bravia HDTV in my bedroom.  I want the sound output to be defaulted to my Sony, via HDMI, instead of the puny, tiny speakers in my Mac Mini.  However, everything single time, after I reboot or boot up my Mini (I don't leave it on 24-7), it always defaults the sound back to the Mini's internal speakers.

Any ideas on how to make my default stay with my TV?  It's very annoying, and it used to not do this.  Any suggestions are appreciated.