the only thing you can do to help this issue is use the volume adjustment on the downloaded files. i've tried everything and that's the only thing that seems to help at all.
get info>options>volume adjusment
apple doesn't use the max audio bitrate (165kbps) in their movies which makes the audio sound a little muffled and compressed. raising the volume adjustment up on the file in itunes will help a little.
turning off dolby digital may help at the expense of losing any actual or virtual surround sound coming through your home theater speakers, sound bar or tv speakers. not a great fix imo but may work for your set up. if you love movies you are really missing out if you don't have a surround sound system!