Antonio unfortunately browsers don't quite work that way.
We're in europe as well, but quicktime is more or less the only option for doing this properly. (well that or flash, and flash also requires a plugin and has a lot of problems...)
If you send users the same link we have in our Sticky notice (the "QuickTime Plugin" link), and users install that, that will work fine in Firefox
I run Firefox myself.
Oh, and PS: Windows Media Player doesn't add a plugin to Firefox unless you install an old and unsupported plugin, so switching over to WMP would ensure sounds only worked in IE, which would be bad