2017 iMac 21.5 Low Sound Output after macOS updates

Hello,

Recently my dad purchased a 2017 21.5" iMac. I believe the model number is MMQA2LL/A. We noticed that out of the box everything worked great. Then he installed the Sierra 10.12.6 update. After the update his volume levels were very low -- almost inaudible. We played around with the volume controls and couldn't get the volume up to audible levels. I helped my dad do an NVRAM reset (command-option-P-R at boot) and voila -- the audio was back, and very loud, sounded great again. Fast forward a few weeks and my dad installed macOS High Sierra 10.13. The problem was back -- inaudible sound no matter what we did with the volume controls. So, I walked him through another NVRAM reset and all was well again.

This doesn't seem like normal behavior. I've worked with dozens of Macs and haven't seen this one. It's pretty annoying for him to have to do the NVRAM reset magic every time he does an update. Anyone have an idea of why updating macOS would kill my dad's audio and require an NVRAM reset every time? Any way to prevent this from happening?

Thanks!