I own a couple of Kingfisher DVD's and they are pretty good with in cab commentary (well, a few people in the cab chatting really) and the sound is alright but the DEV's do tend to be quite old and therefore have lower quality video and sound but because they're older they do contain some great trains.
I definitley agree with Galvanize here though as the Locomaster profiles 'Wired for Sound' series are truly spectacular for sound and high quality video. It'll make you buy a surround sound system just so you can get the full effect of the sound.
Video 125 is best for informed commentary and video 225 are good for quality and sound though.
Hope that helps.