Windows 8.1 has Sonos User Account

That seems pretty unusual.  I wouldn't think installing the Sonos app would add a user.

If you used the Sonos wizard to share your music on your computer to your Sonos players, it will automatically create a user account on your computer. Your Sonos players will use that user account for the purpose of accessing your music on your computer.

Explanation: By default, Windows requires a password-protected user account in order to access data over a network (this can be disabled but it requires some policy modifications and registry edits). Sonos' wizard creates the account with a strong password, then creates a shared folder that the Sonos user account can access, allowing your Sonos players to be able to access the music on your computer.

If you are an advanced user, you could instead opt to set up the account and shared folder yourself, then provide Sonos with the network path and account info that it would need to access the folder.

How can I find out what the password is that Sonos created for this account on my Windows PC? I made the mistake of transferring my PC's administrator privileges to this account from my primary user account. Long story - was trying to fix a broken Windows 10 feature - but now Windows prompts me to enter the Sonos account password in order to make any kind of change to my PC settings. Thanks for any ideas on what I can do to get that password so I can transfer administrator control back to the primary WIndows user account.

Same here accidentally sonos has change my Windows log in password and I've no idea what it should be.