Faux- Since he's a program (and a tiny size wise one at that), I don't think it would be easy to pick up his darkness. But he does have a lot. (he's had a rough time) So if Xehanort could, it would be vaguely a white noise/static of random tones.

[I don't recall in kingdom hearts 2 of the actual programs becoming heartless themselves. (I never played the Legacy portion from the 3ds game) The heartless there were ones that came into the system and took on the digitized looks, so I'm not sure its indicative of whether a programs darkness can be heard, or if its possible for them to become heartless.]