My guess is that your not having problem on Macs but on PC's. Your issue is that you need to use Object tags and Embeds tags to author your content. You are only using EMBED tags. Add the object tags and you should be fine. This will make sure your videos play properly on Mac's and PC's and in different browsers. There are some other recent post here in the forum that will explain more or assist you on your issue. In the mean time check out how I've authored our streaming videos using the Embed and Object tags on our streaming videos.