I can reproduce this in both Google Chrome and Firefox 37 beta 6. Sound works fine outside the brower or in other browser tabs.
I tried clearing the cache
Logging out, then logging back in
Exiting and relaunching the browser
Different browser
In the past, when the sound stopped working during a quiz, changing, then saving site settings or sending an error report got the sound to work again. This time it's site-wide for me.
Hmm - working just fine for me, both browsers you mentioned. How proficient, if at all, are you with javascript? If I said 'javascript console', would that make any sense? If so, I might have you look at something to make sure the call to the data is making it in your situation.
No real proficency with javascript but I should be able to check whatever you'd like me to.
Edit:
I checked Internet Explorer 11, still no sound from Renshuu.
I enabled Shumway in Firefox 37 to see if it would make a difference somehow. Still no sound.
I quit/exited Firefox 37 and installed Firefox 36 Portable but there was still no sound from Renshuu.
I relaunched to my desktop firefox, uninstalled flash and java, then checked the site. Sound started working again in Firefox. Reinstalled flash and java from their respective websites and sound continues to work. Chrome and Internet Explorer are working again as well. In hindsight I was a bit sloppy trying to isolate what could have been glitching my home system, but somewhere along the way I stopped playing beta tester and became just an end-user wanting the sound back on
I know that that's probably more than you expect from volunteer testers but old habits of mine want to reassert themselves and I hate doing sloppy work. I used to do beta and system integration testing many years ago. Anyway, it's really puzzling looking at this from a 'black box' perspective. Both Firefox and Google Chrome support HTML5, flash is embedded in Chrome; Firefox also has Shumway but Internet Explorer uses an Active-X version of Flash if it's installed at all. Plus if I remember correctly, javascript does not equal Java.
Sounds like it was a flash issue, then....very bizarre. I can't wait until the day when we can rely on just HTML5 for the sounds, but we're not quite there yet.