My bet is that instrumental music works best, since lyrics will distract you.
Also, the tempo should probably best be not fast and be steady, again, not to distract you or keep your mind in a more active state.
So my guess is that it has to be somewhat background music, ambient if you will, with no sudden high pitched sounds or instruments that catch your attention.
Sum that up and you'll end up with soft classical music, well that's what I would favor anyway.
A little personal sidenote: maybe it's better to change the thread title from 'IM' to 'Intention-Manifestation'? Or was that too long? I know I didn't get it the first time..)