what would be a good song to put in the audio bacground for a dirt jump type of video?

Something that suits the pace of the riding, so generally fairly slow stuff is used for dirt jumping.
Something you like and isn't too extreme, eg say someone doesn't like a certain genre if you use full on/hardcore music from that genre they more likely to dislike your video, but its up to you.
Also try not to just watch bike films and directly copy the music they use.