After updating to 3.6-RC2, the audio player controls disappear when viewing in Firefox (v22.0) and Opera (v12.16). All you see is a plain black bar, and that's all.
I've just tried this on two seperate sites, both times after updating from RC1 to RC2, the media player controls disappeared in FF & Opera.
The audio player still works ok in Chrome (v28.0.1500.71) and Safari (v6.0.5).
All these browsers were on a Mac (Mountain Lion 10.8.4). The audio file was an mp3, uploaded to the site.
I've disabled all plugins and tried in TwentyTwelve, with the same results.