A long time ago I noticed a weird phenomenon: sometimes if I set an alarm for the morning and went to sleep, in the morning the alarm would go off at the right time but the sound wasn't the sound I had selected, but instead a brief "Bong" which wasn't a sound I could find (I looked because I kind of liked it).
Eventually I realized that this was related to system updates, as follows: my phone is set to download updates but not to reboot automatically. If early in the evening I had set an alarm, and then later at night received a message about a system update, and restarted my phone to install the update but didn't use it until morning, then the alarm would go off with "Bong" instead of the sound I had selected. I thought that was a charming mysterious glitch related to updates, because then I didn't understand about BFU vs. AFU. Now I think what was happening was that the "Bong" sound was an emergency fallback sound stored in the Clock app to be used BFU.
Recently that no longer works. If I set an alarm and restart the phone, when the alarm goes off it shows the relevant cute animation on the screen but no sound is played.
Does this happen for anybody else? Just set an alarm for three minutes out, restart your phone, don't touch it for two minutes (I realize that might be hard!)... then what happens when the alarm goes off?