I just converted my iTunes movies to an open format that I can play on my tablets, and I am having the same audio sync issue, and I am checking my output videos with Windows Media Player on a Windows 7 machine.
In case you didn't already know....you can run ITunes on windows computers so there is no reason to convert. But if you prefer a simpler less "bulky" player try Media Player Classic. It has an option that will allow you to adjust audio sync issues.