i'm sure there will be sound and music like all the other GT games have. The videos were shot in a crowded hall with lots of background noise, so it's not surprising you are not hearing any sounds from the game; be it background music or car sounds.
Those did sound good. Unfortunately the pack of cars going by didn't sound quite so authentic. More like a bunch of sports cars than the savage snarling American V8s. They haven't managed to get the sound for the Corvette C5R right in 3&4, I hope they've managed it for 5.
As for Mobile, I'm sure there will be a full, relatively accurate soundtrack.