I looked through every reference to angels in that book, and I didn’t see anything about them being able to read our minds. What I think happens is that, although they can’t read our minds, they are intelligent enough to discern what we are thinking to some degree of accuracy. They observe us and know when we sin, so it’s relatively easy for the fallen angels to figure out the best ways to tempt us.
As far as angels/saints being able to hear us differently if we pray aloud or mentally. Angels don’t have bodies, and neither do the saints in heaven until the resurrection of the body (except for Mary, Enoch, and Elijah). Our ability to hear audibly is a physical trait, so a saint or angel without a body would still need God to reveal to the angel or saint what was said aloud. The symbolism of praying aloud to indicate that only God knows our hearts may be useful, but I don’t see any reason why an angel or saint should be able to hear audible prayers but not mental prayers.
