XBMC video with no sound issue

Hi, I'm relatively new to this forum and very new to using XBMC.  I was using Nito but I had way too many issues with stuttering so I switched to XBMC which seems to be running smoother.  However, some of my video files from my NAS now don't have any sound playing.  Which is weird because there was sound playing when I used Nito. This is occurring mostly with avi and mkv files.  However, it has happened with an mp4 file as well.  I've tried looking for answers but haven't seen anything so far.  Any help would be greatly appreciated.  Oh, and I'm running the latest version of atv flash and XBMC, connected wirelessly to my Drobo FS via an airport extreme.  

An update.  I think I may have resolved the issue, unfortunately, not with XBMC.  I've downloaded the latest version of Boxee and that seems to be able to play almost any file type I've thrown at it (mkv, avi, mp4), and play with sound via my receiver hooked up to my atv with coax!  Also, this seems to have the least stutter issues as well. I still don't understand why it should work with Boxee but not with the other media players included with ATV flash.  if anyone could explain that to me I'd really appreciate it as I'm trying to learn as I go.  

Same issue with xbmc and boxee. Any fixes

UPDATE : I had the Same problem with apple tv1. Went to apple tv settings>audio&video> and turned off Dolby digital out. Seems to have worked. I did not have to add the BCM70015 video card.-- this was done in xbmc & boxee. I also put the video settings to 1080p.