No Audio on HD ripped movies on ATV3 with v5.1

I really enjoy streaming movies and the like from my iTunes library to my three Apple TV's (2xATV2 and 1xATV3). I have ripped and encoded much of my DVD collection and once i got around the audio channels and the fact that i need to have the primary as Stereo I haven't looked back. I use DVD ripper on an old PC and then create the MP4 file with Handbrake on my iMac......fantastic.

recently I have started ripping and converting my Blu-Ray collection into HD mp4's using Pavtube on my iMac. Again i seem to have no problem creating and watching them until recently i.e. Sunday when I upgraded my ATV3 to firmware v5.1........I now get no audio from HD files that i have ripped but no issue with DVD's or downloaded HD content.

My iMac plays the movies fine through iTunes, they play absolutely at 720p on my ATV2's (still on firmware 5.0.2) it's just my ATV3 where i have the problem.

So I have a few questions: -

1. Has anyone else experienced this tedious phenomenon?

2. Is it possible to downgrade the firmware upgrade to v5.0.2?

This isn't the first time that i have encountered oddities with firmware upgrades and an apparent lack of regression testing, but I love the products and what they offer when working. My kids and my wife can make it work! 😉

Thanks