How can I resolve HomePod 2 won't factory reset, plays music randomly at high volume, and Siri talks by itself.

This was hard to follow.

When you factory reset you do need to set it up again because it’s been erased. That has not changed. See: Reset HomePod or HomePod mini - Apple Support for further assistance on Resetting.

This is a common issue, Homepod 2gen's touch panel is faulty and some will randomly act up. You can add touch delay through home app, some say it helps with the problem. However evnetually it wont work anymore, and the ultimate solution right now is to open the homepod and disconnect the touch panel connector, basically disable the touch panel all together. I have 2 homepods and had to do it to one, was a quick 15min job. Have a needle and thread ready cuz you will need it to tighten the mesh covers when you put it back.

Sounds like you’ve already tried most of the usual fixes. If one HomePod still won’t reset and doesn’t show up in the Home app, it may be stuck in a software fault. You can try leaving it unplugged for at least 24 hours, then plug it back in and hold your finger on the top for about 10 seconds until you (normally) see the reset prompt — even if Siri doesn’t speak, wait to see if it reappears in the Home app as “new.”

If that doesn’t work, your best bet is to contact Apple Support directly for hardware service, since this could be a deeper firmware or logic board issue. In the meantime, keeping them unplugged is the safest way to prevent the random loud music problem.

I am facing the same issue with my HomePod 2, the Apple Store in Amsterdam is completely clueless, I contacted Apple Support and got again, friendly, sort of a denial and sounded like I am the only person on the planet with this issue. A common mode of reaction by Apple support. It is really super annoying and especially when it starts playing in the middle of the night. It is time Apple comes out with a statement and a solution, if the touch panel turns out faulty they should do a recall, if it is a software issue, let them give us the option of software rollback to a stable version. I bought an expensive speaker from a trustworthy brand I thought, but turns out to be a lemon with some delay...

i did removed the touch panel cable, but my homepod play music again after io18.5 update

Same problem with HomePods playing music randomly. I unplug my HomePods at night. Nothing seems to alleviate the issue.