Does the default mail sound work properly? Maybe your sound file is too big/long? When does Thunderbird check for e-mails? Maybe the sound doesn't play when you click the 'get mail' button.

Oooh! No, I've just changed Thunderbird sound settings and the default sound didn't play when I sent a mail message from another mail provider (arcor) to my Thunderbird account. My local .wav file is 175 kB and 16 seconds long.

A wav file that's compatible with TB has to be 16-bit, stereo, 44kHz. If yours isn't that format, there's a free online converter here.

A 16-sec. wav file in the correct format would be about 2.5 MB.

Poking around on Google, it looks like this is a somewhat common problem, especially on Linux. It seems to have something to do with the audio drivers. Try using a software like Driver Booster to make sure your audio drivers are all up-to-date.

Thanks sfhowes! I used the free online converter to update my wav file, copied it to my desktop and directed TB to go to that location to find my local wav file. It is now 2.74 MB. An incoming mail from another PC still did not trigger the local wav file.

I will now take advice of SuperSluether and try audio driver software updates. Will post a reply when updates have been checked.

When you select a custom wav file for the sound alert in Tools/Options/General, click the Play button to make sure it's audible.

Sound problems can sometimes be fixed by removing the sound device from Control Panel/Device Manager and restarting the PC; the sound device will be automatically detected and installed.

If it turns out the sound system is working, see this post that applies to IMAP accounts such as Gmail:

https://support.mozilla.org/en-US/questions/997744

An update on local .wav sound file for incoming TB mail messages.

Driver Booster found outdated drivers and all have been updated.

Sound device has been removed from the device manager and automatically re-installed.

From advice given I have turned mail.imap.use_status_for_biff to "false".

Still no sound from my local .wav file. Is my problem unique? Is 2.74 MB too large as a sound file?

This problem isn't unique. This problem has been around since V3 of Thunderbird. If the default sound isn't playing, I'm at a loss for what's happening here.

Thanks for trying, SuperSluether. Having just switched my PC on and opening TB, your mail arrived and my default sound actually worked. It seems that any further incoming mails do not trigger either the default or the local sound source.

Has anyone any further ideas? :-)