As other commenters mentioned, it requires flash for sound. Any flashblocker or clicktoflash will render the page mute (like on my Safari). Maybe you have one of those installed?

Ah yes, Flashblock. I didn't see the little icon on the page, so it didn't cross my mind.

Thanks for pointing that out.

It works in my copy of FF4 on Win7.

Firefox 4.0.1 on Win7. No sound. Chrome works fine.

Works on Firefox 4 for me.

I've tried it on FF4 on both Linux and Mac OS X. Works just fine on both.