From Gnash Project Wiki

There's an old standing bug preveing Audio and Video to be in sync in some cases. This is usually due to missing synchronization mechanisms between the animation playhead thread and the audio mixer thread.

More info in this page are welcome.

Examples of movies with good A/V sync

• http://www.freaknet.org/martin/video/Bozzetto/Bruno%20Bozzetto%20-%20Female%20and%20Male.swf

Examples of movies with bad A/V sync

• http://www.freaknet.org/martin/video/Bozzetto/Bruno%20Bozzetto%20-%20European%20Italy.swf

References

• http://www.b-nm.at/flash-sound-synchronisation/