It stopped for a while then started after rebooting . That it doesn't even care if I've muted my speaker is annoying the hell out of me . I'm going to start looking for an alternative to Thunderbird .

I'm wondering if it has anything to do with 'Play a sound' when new messages arrive as this is the only sound Thunderbird allows if option is selected. But that should only work when new messages arrive.

Not unless the unread messages are only headers as can occur if imap account and so the sound occurs on download and this is triggering it.

I must admit that it does sound more like something your computer has as an option.

In Thunderbird: Try unchecking the following: 'Menu icon' > 'Options' > 'Options' > 'General' When new messages arrive: uncheckL 'Play a sound' click on OK

If that does not effect anything then it is not Thunderbird.

Otherwise this is something set on your computer for an onclick sound. In Control Panel Sounds or Hardware and sounds - depends on OS Under Sounds - select 'Start Navigation' Sounds: select 'None' click on 'OK'

It may also be your microphone. In microphone properties and unchecked 'Listen to this device' Did a search in google and found this which mentions several different methods depending upon what is triggering it. https://answers.microsoft.com/en-us/windows/forum/windows_7-pictures/sound-with-every-mouse-click/9d70defd-5679-44f1-92e2-62d20abfbcaa?auth=1

Thank you very much for your answer . But the beep goes on .

I don't mind the "new mail" sound . It's when in the morning I'm going thru 30 new msgs and it gives a 2-tone on every unread one -- and muting sound ( Win 10 ) doesn't stop it -- that drives me nuts .

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I turned off the new-mail sound and the next time I rebooted , and not until then , the sound stopped .

So I have it under control now anyway . There do seem to be bugs in that area .

I think your choice here is to try the following "tests". Thunderbird is an application and has no access to the computer speaker at a low enough level to actually bypass the windows setting. So something other than Thunderbird almost has to be the cause. Previously I had issues with my external speakers that made odd noises and it was the speakers that were failing. Other programs that routinely have very low level access to the operating system are anti virus products and hardware drivers. So it could be your mouse driver for instance.

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.