Syncronization of sound to an animation that's heavy on the graphics

Wondering if anyone can offer any advice. I've decided to satisfy a goal for my own personal gain to make a game using flash.

I'm utilising packages like Photoshop to create images and animating them in flash. I have a large stage area (1024x768) running at 50fps. It is 600 frames long i.e. 12 seconds. I have two layers of mist covering the stage scrolling in opposite directions over the duration of the scene, the result looks stunning however can't help but notice that it's crunching into the processing power to play the movie back. I added a 12 second long ambient wav file and consequently the sound is finished long before the graphics.

Is there anything simple that I can do (not buy a better PC:)) to rectify this or am I going to have to tone down the mist effect?

Thanks in advance

You would need to create a preloader, which would load all of the graphics and sound BEFORE it plays. Then everything would start at once. There are tons of tutorials out there but here is one I found that covers a lot of information:
http://www.senocular.com/flash/tutorials/preloading/

PS: your settings are pretty high. Normal TV doesn't even use 50fps, it uses just under 30. Web is more like 12-15 on average. Why? Its usually an issue of filesize. I don't know how large your file is but I bet its huge. So I would also suggest pulling your frames back to at least 30 or less, and add the preloader. Otherwise you would crash some browsers.

Thanks for this, I've created a preloader a couple of times before for earlier projects, but they only 2 scenes so to speak, the preloader and then the main movie. This one already has 2 scenes and it's the second one that's the beefy one. I will reduce the fps but can you put a preloader in before scene 1 and get it to load both scenes before playing them both?

Your preloader should load the ENTIRE file, not just the first scene. Just make sure that the preloader uses the overall filesize in determining when it has been loaded.