I was documenting the Playlist Builder in 1.7.0-alpha and noticed the
play/pause button for each row in the playlist. This doesn't play any
sound for me on Iceweasel 3.5 (default browser in Debian Squeeze). Is
this one of the features which requires Firefox 3.6?
When previewing an mp3 file there are 2 fallbacks. First if the browser supports mp3 playback natively via the html5 audio tag, then this is used (chrome and ie). In the case of firefox which only natively supports ogg, we fallback to a flash solution. In your case you probably are missing flash so there is no fallback. Ogg files should work for iceweasel/firefox, unfortunately, there is not much we can do for mp3 files.