Apple iPhone 7, iOS 13.7. (iOS 14.0.1 to install)
Apple AirPods Gen 2.
I just experienced this issue for the first time last night when sleeping and then again today while walking in an otherwise very quiet park at twilight surrounded by residential homes – so not good timing both times. I can't remember how I stopped it the first time, but didn't have the AirPods in the case. One was on the bedside shelf, and the other somewhere in my bed. Perhaps an AirPods bug triggered it if my body pressed against it - making it presume it was in my ear.
Just before the second time, I had invoked the Find My app to locate the AirPods, but the sound didn't play on them. The Find My app showed the sound was 'Pending'. This Find My app might not be relevant since it first occurred the night before I invoked it. Although, there is a difference: The first time I could stop the sound. For the second time, the AirPods were cased and in my pocket but I couldn't stop it till I returned home and turned off bluetooth – an inspiration from reading other users' experience. I did play with a bunch of other settings but suspect none of those made any differnce. Just like other users' experiences, I had no control over the auto-playing nor volume. It did seem that the phone in general was responding very slowly – that is, any adjustments to volume etc could be seen to occur in sequence about 30-60 seconds later, but ultimately the auto-play would continue and revert to full volume. I also note that the remaining battery power percentage was visibly dropping relatively quickly. At Settings => Battery the top power user apps had nothing to do with the issue (Last 24 hours showing top 3 as Quora 22%, Notes 13%, Measure 9% in my case).
In my case I had just recently started using Apple bluetooth Air buds (generation 2), so I expect the issue is caused by this bluetooth paired device. I don't use Pandora (not installed), nor Spotify (installed but never used recently). The only music app I use is Apple Music. I do often use the sound-related app myNoise which was probably open the first time, but not playing anything. The second time it wasn't playing and probably wasn't even launched. In either case, the music was being played from Apple Music.
After turning the bluetooth back on, the issue has not reoccured (1.5 hours and counting).
I'll install iOS 14.0.1 tonight. With hope, it will prevent further instances of this issue.