I'm on Mac OS X 10.8.2 running Firefox 18 and I'm not able to get the HTML5 Soundcloud player to work. I get blank space where it should be, and "Additional plugins are required to display all the media on this page" shows up at the top of the page. If I click that, it prompts for Macromedia Flash.
I'm working locally, but the same is true here:
http://blog.soundcloud.com/2011/11/03/html5/
I don't have any extensions, so I'm not sure what the issue could be?
Per some advice, I added an htaccess file with the following, which didn't work:
AddType audio/ogg oga ogg
AddType video/ogg ogv
AddType video/webm .webm