Q: Mac mini (mid 2010), Sierra upgrade, sound not working

I upgraded my Mac mini (mid 2010) to Sierra 10.12, and the output sound via the headphone / optical digital-out port has stopped working. However, oddly, this output does work when the video display is asleep.

I usually have the sound outputting through the optical digital-out port, to the digital in port of my Yamaha DVR S300 sound system. The video output is via HDMI to a Sony Bravia Color TV (KLV-40S220A). It is an old setup, but has worked well for a number of years. This problem has only emerged with the upgrade to Sierra.

Steps taken so far:

• Rebooted the Mac mini - makes no difference
• Tested the headphone / optical digital-out port, by swapping the digital-out connection to some analog headphones - no sound output
• Changed the audio output of the Mac mini to HDMI - this works; the sound outputs through the TV

I discovered by accident that the optical digital-out port does work when the Mac mini puts the screen to sleep. So it's possible that there is a setting that disables the optical digital-out port when there is an active HDMI output. However, I am stumped and would appreciate some guidance as to how to rectify this issue.