Nice!
Very good for a first animation. I can tell you put alot of effort into it, and I think you chose the most fitting song for the credits. The sounds were well done, and the voicing wasn't bad either.
As for your audio problems, try the following:
Select a frame that holds the sound. Then look below the stage (the place where you draw everything), and click open 'properties'. At the drowdown for 'Sync' select 'Stream'.
With the sound set to stream, it will ensure that your sound never gets out of sync when viewed on the internet :)