I’d generate a diagnostics report, then unplug the unit from power. Then contact Sonos support. As you say, it sounds electrical so there’s a fire risk if something is shorting out. A visual check of the power cable is also worthwhile.
You may be able to work with Support to get a discount on a new Sonos if you haven’t already claimed the credit on the Play 5.
Did the diagnostic report a ewrlier today. Will get in touch with Sonos next week to hopefully having this resolved somehow. It’s an unlucky speaker, Sonos replaced it (free of charge!) once already. The first one went dead on me quite soon after purchasing it.
PLAY:5 has been a reliable product. Your initial issue was an outlier.
Some warming in the back is normal.
This current issue is likely age related. While I don’t recall any previous similar reports, this may become a standard failure mode for old units. Service centers can often tell the customer what the problem is after given only the model number. This does not imply that all units will fail, only that those that do fail often have the same issue.