I could put any words up to that music and you'd think thats what it was saying
So correct. Because our brains are so used to hearing noise or sound with meaning behind it (words, etc.) When we hear foreign languages, and mumbling nonsense, we try to put meaning behind it and hear what we what so there is meaning. In this case we play a song backwards, it says mumbling junk and our brains try to make meaning out if it and apply words that sound similar to what your brain is hearing. I forgot what else you could say, but there is another sentence that you can say at the same spot as "Highway to Heaven" and It sounds exactly like it.
All it is is our brains being themselves.
