I'm using Firefox (3.0.7) on Windows Vista Ultimate x64 and I don't hear any audio when it's playing through Firefox. Youtube vids play but I get no sound. Sound works in other programs. Any ideas?
On your Vista taskbar, double click the audio icon (while a youtube video is playing in Firefox). A window should appear with volume sliders for programs including Firefox. Just slide the one for Firefox up to increase the volume, and make sure the mute box isn't checked.