The direct playback with USB by the TV gives awful sound. Like you, I also experienced the same thing, near negligible sound from the TV. I think there is some issue with the playback which can't reproduce or amplify the sound from movie files. Though I have tewo movies one with a dubbed soundtrack and another with a dual soundtrack. For both Hindi soundtrack, the volume is good, but for English soundtrack, it refuses to amplify.
BTW, the same audiotracks works fine, if you are using a BDP to play it. SO it seems there is some issue with the media player given with the TV.
Another point is that overall the TV sound quality is Average or less than average. In fact my old Sony LCD speakers were better.
All Samsung irrespective of lcds,leds,plasma have shit of audio quality.Need to go for hometheaters.I have one 32inch samsung lcd when you hear audio from tv , my old thompson tv has better sound.