I'm on a MacBook Pro (Retina, 15-inch, Mid 2015) running 10.11.3 (15D21). I haven't changed any sound preferences. I bought the notebook brand new about 4 months ago. I only have a few apps installed (namely, Firefox, Affinity Designer, Sublime Text 3, and Transmit from Panic), so nothing specifically sound related, and all very common apps. I haven't changed any sound settings to cause this issue (at least not intentionally).
Before the issue
This is how the notebook has sounded up until today (watch @3:27): https://www.youtube.com/watch?v=vOQOcCISgMI&t=207
After the issue
Today, the volume indicator was suddenly muted (not just SHIFT + 🔊 one-time muted, but permanently muted). I went to sound preferences and checked the
Play feedback when volume is changed box to re-enable it, but now my volume indicator sounds like this (same video @0:15): https://www.youtube.com/watch?v=vOQOcCISgMI&t=15.
I'm not as concerned with switching back to the old sound (which is easy, via this and many other videos). My main question is why / how did this happen (the indicator change + muting)?
I did update to 10.11.3 on the 20th, and it's quite possible that I haven't changed my volume since then and simply did not notice that the 10.11.3 update included these changes (hopefully, this is the case), rather than it having just muted / changed today. I can't seem to find a detailed change log for this release though, to confirm that.