Using easysound.swf as a testcase (you can find sources of it in testsuite/misc-ming.all/easysound.as) you can see that while FFMPEG media handler succeeds in Load/Start a non-streaming sound GST fails there.

In particular, with GST you get onLoad(false) called while with FFMPEG you get onLoad(true).

This is as of 6b478714a7264187cce8bd6d24c458ed94441843

Logs:

 25942:1] 10:14:09 DEBUG: MediaParserGst/typefound: Detected media type application/x-id3
 25942:1] 10:14:09 DEBUG: Needed 0 dead iterations to detect audio type.
 25942:1] 10:14:09 DEBUG: Starting MediaParser thread
 25942:1] 10:14:09 TRACE: onLoad(false)

The only way for onLoad(false) to be called is from Sound_as.cpp:

         if ( _mediaParser->parsingCompleted() )
        { 
            _soundLoaded = true;
            if ( ! isStreaming )
            { 
                stopProbeTimer(); // will be re-started on Sound.start()
            }
            bool success = _mediaParser->getAudioInfo() != 0;
            callMethod(&owner(), NSV::PROP_ON_LOAD, success);
        }

So I suspect MediaParserGst::parsingCompleted() is returning
true while MediaParserGst::getAudioInfo() is returning 0.