OK, first of all... this is the same cart you were having trouble with before right? Did you ever open the thing? When you brought it back to him did he put it on a Neo there to show you it was working 100%? I had a King of The Monsters and Pulstar carts that had scratchy sound, after cleaning the contacts they were fine... If you have already cleaned the contacts like I suggested before (art-gum and compressed air) then this cart is done for... especially since your board plays other games fine. Are we 100% sure this is not a bootleg cart? Remember, most bootlegs have bad sound. I really doubt it is the Neo board itself, I have never had to clean my MVS board but I did blow it out with compressed air once (Not for any reason other then I was dusting the inside of the cabinet).
-Nick