Smarter Every Day has posted a very nice example video of audio master Gordon McGladdery recording audio to be incorporated into slow motion footage. Since recording real slow motion sound that is usable at super high speeds is an impossibility due to the undiscernable pitch that results from it, you need to get creative with some reverb, echo, pitch, and chamber audio effects in order to get a compelling final product.  We are big fans of both their channels and make sure you check them out for supporting them.