The voice of the Sorting Hat came to me quickly. In years past there was an animated Moses movie, which I didn’t bother to watch, but I managed to hear an interview with someone involved in its production. They were speaking about the difficulty of coming up with a decent Voice Of God during the Burning Bush scene. The “old American patriarch” voice certainly wouldn’t do, because this movie was aimed at the broader public rather than just fundamentalist American Christians. But how do you do God’s voice then? You can’t use a male or a female voice without assigning god a gender, you can’t make him sound like any particular nationality or ethnicity, etc. They tried layering many voices of many actors together for an all-inclusive deity voice, but apparently it ended up sounding awful.
Eventually they settled on simply taking Moses’s own voice and echoing it back to him. Which I personally thought was brilliant, and appropriate on several levels (pssst.. He only exists in your head! ;)) So when Harry is talking to a very powerful entity that borrows his own brain hardware in order to extrapolate what his sorting volition would be… well hell, what it should sound like was easy!