Ok, tried with a fresh profile and got the same issue.
Here is a screenshot for the debugging console. Not sure what it all means…
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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 everything…
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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I’m 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 I’ll get back to you when I find out more!
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Trying to get to the ground of this… So I have another request of something for you to try. Could you please go to about:debugging and click on “Manifest URL”. Then change the “manifest.json” part of the url to “pop.flac”. 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 doesn’t work with the short MP3 I was trying to use for the notification sound. I haven’t 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 that’s 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 it’s 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 won’t 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 don’t 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?
Your location does not matter, so I’m a bit confused about this question.
Thanks, good follow-up.i used google tranlate to speak with you sorry.
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