Audio synch in videos causing a real headache in SL2

HI everyone, 

I am an experienced SL 2 developer in Australia and have together with my programmer developed several video based courses in SL2 for online training. We usually host the video externally and bring them into the project via Vimeo but in recent times was asked to bundle the video in the project output for delivery into a clients system as a whole instead of web hosting of video  The videos are mp4's compressed using handbrake. we are finding very regularly that the audio is out on videos within the SL2 output bundle and this is causing us some repetitional heat and distress. We have researched extensively but still cannot find any reason why this is only happening when we export the content within the output bundle instead of getting it off Vimeo where all is fine.

If anyone can recommend a solution that would be very helpful. I am encoding the videos at around 720p with data rate of about 1.8-2.5 range to keep the files smallish. typically we are finding the videos are about a second out of synch. we are also finding the videos are waiting to download all the videos in a slide before playing any of them on some browsers (chrome - FF) but running straight away on safari and iPad!

If you can let me know where we are going wrong, that would be great and make sure you all vote too. we are all counting on you guys to do the right thing!

Love and regards form Australia !