When I'm at a site I enjoy the quiet and the noise of Nature. I sometimes think about music but I'd never take a Walkman (I know, I know...) or an Ipod or anything. For me, a site's geographical setting and sheer physicality suggests music of a massive creative force played loud and hard. However, a site's great age (whether a tomb, a hillfort, a stone circle, some Rock Art or whatever) suggests more peaceful, serene themes to me. So I kinda flit between a full blown Led Zep and a harp plucking Robin Williamson (who sit very well together on my in-car cassette-deck system)! The ideal track could be "The Wee Thing" by Chou Pahrot...
When I'm travelling to sites I find that I mostly favour blissed-out ethereal ambient soundscapes. Such as

http://www.youtube.com/watch?v=rQiru402Bps&feature=related

On my return journeys I often find myself drawn to long, loud, primal instrumental guitar workouts like those of Glasgow's Finest Post-Rock enthusiasts - the mighty Mogwai.

http://www.youtube.com/watch?v=yUaCxx5npko

And Mr Cope's "Jehovakill" is never far away...