Can you embed the music in the flash file and make it play through flash?
Set your sound to streaming. This will cause the sound to attach itself to the timeline and play the same spot per frame everytime.
Scrub across your timeline and you can hear the individual audio packets.
You can then re-time the visuals by insert/removing frames at various point in your movie, to match the sound layer.
Now, why anyone would want an put an intro on a web page is beyond me, they are nothing but a waste of time for the end users and a waste of bandwidth on your behalf. Theres been books writen on why you shouldnt use intros so Im not gonna try and explain it all here, butif you absolutely must have something I would recomend a TV-style bumper. No more than 3 seconds long. Something that enables a transition into your home page, withough becoming a mind numbing bore and still reinforce your branding.