I think Verlyn Flieger's 'A Question of Time' maybe relevant. Basically, she explores Tolkien's use of Time, & the influence of the theories of JW Dunne.
http://www.greenmanreview.com/a.question.of.time.htm
There are numerous incidents throughout the Legendarium of Characters experiencing future events - Frodo' dream of the Undying Lands in Bombadil's house, the same thing he sees at the end of the book, or Aragorn at Cerin Amroth, feeling himself both there at that moment & at the same time back with Arwen.
Its as if Frodo's higher/spiritual self is able to 'look down' on his life & percieve it as a whole, & thus to 'tune into' any particular event (like an observer looking at a landscape from an aircraft, seeing a complete river in a single moment, whereas a traveler in a boat would experience the journey down river as a process/movement in time. So Frodo's 'vision' of the Undying Lands is not a vision of a 'future' which hasn't yet happened, but a glimpse of an event which in some sense already 'is'. Perhaps all the 'prophecies' & visions of the future throughout the Legendarium are of this kind.
How this relates to the Music of the Ainur, which is playing out throughout the history of Middle Earth is an interesting question. The Music sets out the course of events in the world. Water contains echoes of the music. The Music is somehow accesible to those sensitive enough to tune into it. So, prophecy in M.E. is due to certain sensitive individuals 'tuning' into the Music, or its echoes, through dreams or visions. Or, perhaps Illuvatar grants these visions, because Illuvatar would have knowledge of all events throughout the history of ME. The Music at least makes prophecy within ME logically possible.
<font size=1 color=339966>[ 9:24 AM January 16, 2004: Message edited by: davem ]