And this is certainly on the latest version? How familiar are you with developer tools (linked earlier in this thread, too).
If not, could I ask you to try it on a new profile (use -p when launching Firefox)?
Latest version of Firefox (checked updates to be sure) and Iām pretty sure the latest version of Notification Sound, whatever was available here: https://addons.mozilla.org/en-US/firefox/addon/notification-sound/ . I will try with a new profile and if that doesnāt work, I will see what it says in the debugging console. Will report back shortly.
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Ok, tried with a fresh profile and got the same issue.
Here is a screenshot for the debugging console. Not sure what it all meansā¦
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Seems like it can not identify the MIME type of the audio file. Do you see the same warning when choosing an MP3?
Same everythingā¦
I was using this addon fine yesterday and for many weeks, but I reinstalled Windows several times yesterday due to a perceived issue with a game and after the reinstall, this addon never worked again.
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Iām looking into possible ways to fix this on my end, but there is likely some issue in your installation that prevents it from properly detecting the MIME type here 
Thank you for your investigations and Iāll get back to you when I find out more!
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Trying to get to the ground of thisā¦ So I have another request of something for you to try. Could you please go to about:debugging and click on āManifest URLā. Then change the āmanifest.jsonā part of the url to āpop.flacā. Does it play the default sound?
It showed this error 
so I installed VLC and the sound plays at the requested Manifest URL now, but still functions the same in the addon itself (same error as above).
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Does it play using the VLC plugin? Thank you for confirming this, btw!
Actually I restarted Firefox and it seems to be working now, at least with the default sound. Still doesnāt work with the short MP3 I was trying to use for the notification sound. I havenāt tested if notifications actually make a sound yet or not.
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The custom notification sound does play in VLC just fine. I tested with the default sound and the addon does make a sound for notifications.
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Thank you for testing this!
As far as I know all Firefox versions should be able to build FLAC files, so thatās really weird. And as we have established, Firefox can play the MP3 file directly, however it somehow fails to properly play it in the extension context.
Yeah itās kind of a strange problem to me because it was functioning just fine before I did the reinstall yesterday. So I just converted the MP3 to FLAC and put that in as the custom sound. No error showed up, sound played in the Options. Tested the notification sound and it worked! So, for whatever reason, MP3 wonāt work this time around but converted to FLAC it works just fine.
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I have a 64 version in firefox, the sound does not sound when the site alert is just showing me the text below my screen. I have the default sound and I have made and I have a sound flac. I have to do;
Does the sound play if you click the test button in the options?
Yes , are play default sound and the other who make .
Right, then either the website explicitly marks the notification as silent, or uses a means of notifying that the extension can not catch.
I have selected 4 news sites and no one sounds out, what should I do to get the alert as silent?
Yeah, news sites likely donāt work. As is explained in the add-on description, notifications that are also sent while the page is not loaded in a tab can not be intercepted by the extension.
I am from Greece, do we not have to support this extension?