Hi. I believe I have your solution. (Skip to *** below for the ;TLDR version).
I was noticing something similar. What I had noticed was that the "ding-ding" when a heart rate reading completed was no longer playing. This left me scratching my head and I tried a number of things to figure out what was wrong. Nobody had any answers.
I found this thread and saw you mention missing sounds, like the ding when using Google Now. I checked and sure enough that sound was missing on mine too. I thought perhaps a recent app I installed might have been the culprit. I'd recently installed the Chromecast app and also Netflix.
You mentioned that when you went into some radio app that it would fix the problem. This gave me an idea. Before I uninstall recent apps why not try running one to see what happens. I loaded Netflix and played a video. And.... No sound from the video!
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Realizing I had been playing with the volume controls while playing around with my new Chromecast, I must have turned the media sounds all the way down! I turned up the sound in my Netflix video, then went to check the missing sounds (like Google Now and the ding-ding from a Heart Rate reading). They were back!! Woo Hoo!
I recall there being different volume adjustments for various things on the phone, but they weren't to be found in the Sound options screen. Though I recall having seen those when using the volume buttons at one point. I checked, and noticed that when you use a volume button you will see a little gear on the right hand side of the volume indicator. Click that and it will show you all the various volume levels for different things on your phone, including media. If that is all the way down you will lose many of these sounds in apps that we were noticing.
Hopefully this solves your problem
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