In the raw Blu-ray rips of Season 1, the secondary language tracks (the "Dual Audio" part) were often encoded in AC3 or DTS formats. The glitch occurred because media players like VLC sometimes failed to properly downmix these surround sound tracks into stereo (two-channel) audio for standard speakers or headphones. If the player didn't recognize the center channel mapping for the secondary language, the characters on screen would move their lips in silence.
To this day, if your Season 1 files play perfectly in two languages, you owe a silent toast to the man who mended the rift in time. Game Of Thrones Season 1 Dual Audio Fix