MacBook Air volume low

Have the same problem with my MB pro. It is annoying to keep restarting. I installed eqMac that would let me add extra boost to the volume when this happens (pushing all the EQ frequency sliders up). It's not a very pretty solution and Apple should really fix this, I notice quite a number of funny bugs like this with Sierra that I don't experience with the previous version, I wonder what is happening with their software QC ?

That's a strange problem to have. What were you playing? Was it music or a video? What app were you using? If you tried to play music with a different app, would it play at full volume?

When this happens, go to System Preferences > Sound and see if the output slider has changed from where you set it (either by dragging or by setting the volume with the volume keys). If the volume slider is staying steady even when the volume drops, that would be strange — but then it would be if the slider moved by itself too.

Does this happen when the Mac is quite warm, or is it typically cold or barely warm to the touch when the volume drops?

Thanks. but unfortunately restarting & reseting NVRAM regularly is not very practical.

Try a NVRAM reset:

https://support.apple.com/en-us/HT204063

Ciao.