Comment 59 for bug 732572

(In reply to Chris Coulson from comment #45)
> Which sounds don't work? And in Thunderbird or Firefox? The error you're
> getting actually comes from GTK when it loads the required GTK modules at
> initialization (as specified on the xsettings manager window), long before
> we do anything else with sound.

On both Thunderbird and SeaMonkey I get the Gtk-Message, but not on Firefox.
As far as I can tell:
Firefox works for both .play and .playEventSound
Thunderbird and SeaMonkey silently fail for .playEventSound and give for .play the following message in the error console:
Timestamp: 15/01/12 21:08:23
Error: uncaught exception: [Exception... "Component returned failure code: 0x80040111 (NS_ERROR_NOT_AVAILABLE) [nsISound.play]" nsresult: "0x80040111 (NS_ERROR_NOT_AVAILABLE)" location: "JS frame :: javascript:%20Components.classes["@mozilla.org/sound;1"].createInstance(Components.interfaces.nsISound).play(Components.classes["@mozilla.org/network/io-service;1"].getService(Components.interfaces.nsIIOService2).newURI("file:///usr/share/sounds/pop.wav",null,null)) :: <TOP_LEVEL> :: line 1" data: no]

So it seems that comm-central based projects are maybe not initialising something that Firefox is...

I've built SeaMonkey, Thunderbird and SeaMonkey from the same source tree.