Works fine here. The website can play a .ogg file and a .mp3 file.

Can you open these in a Firefox tab?

<audio>
 <source src="http://static.sfdict<i>.</i>com/audio/lunawav/C07/C0714400.ogg" type="audio/ogg">
 <source src="http://static.sfdict<i>.</i>com/audio/C07/C0714400.mp3" type="audio/mpeg">
</audio>

If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block (wanted) content (e.g. Adblock Plus, NoScript, Ghostery, uBlock Origin) always make sure such extensions do not block (wanted) content.

• make sure your extensions and filters are updated to the latest version

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

You can open the Web Console and check for messages about loading media content.

• "3-bar" menu button or Tools -> Web Developer
• check on the Network tab if content from specific domains fails to load

• https://developer.mozilla.org/en/Tools/Web_Console

Thanks for your quick reply cor-el.

I didn't know how to open the file you asked, i.e., <audio>

I had to open as http://static.sfdict.com/audio/lunawav/C07/C0714400.ogg and http://static.sfdict.com/audio/C07/C0714400.mp3 and they are both ok (audio file works).

I then open Firefox in safemode as suggested and it works. Back to "normal" Firefox with AdBlock disabled everywhere still no sound. I also disable hardware acceleration but no luck.

I will remove AdBlock completely, restart Firefox and keep you posted. Thanks!

I remove AdBlock, verify that theme is default, restart Firefox but still no sound from http://www.dictionary.com/wordoftheday/

Currently, there are only 3 add-ons as in attached picture: Dictionary Extension, Netvideohunter and Zoom

Any other suggestions? Thanks in advance.

You can try to disable hardware acceleration in Firefox.

• Options/Preferences -> General: Performance
  remove checkmark: [ ] "Use recommended performance settings"
  "Use hardware acceleration when available"

You need to close and restart Firefox after toggling this setting.

• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

You can check if there is an update for your graphics display driver and check for hardware acceleration related issues.

• https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

I turned off "Use hardware acceleration when available" and restart Firefox but no change. Interestingly, when I'm at http://www.dictionary.com/wordoftheday/ I can't get the sound working but when I go to the root domain http;//dictionary.com then sound is working fine using identical Firefox settings.

The original problem isn't fixed but for all intents and purpose, I'm able to get the pronunciation of word-of-the-day by going to http://www.dictionary.com.

I consider my question answered. My special thanks to cor-el fro trying to help with this question. Best regards!