Ghost morganizedchaos : Go for it! That said, his darkness would probably sound distinctly... un-sound? His kind of alien is canonically associated with the space between the stars so the like. "you can't hear anything because there's nothing there to hear" of vacuum, and any whispers Xehanort might get are going to feel like they're within a bubble that's the equivalent of the comms line on a space suit helmet. ... I'm sorry he's complicated.

Asch oncedriven : First of all, Asch is a dark-presenting blade which while that does mean darkness in a more literal sense than the emotional one of the KH series, it's also associated with the virtue of justice. That, as well as Asch's personal hyperintuition, are going to be consistently prominent in whatever Xehanort might get off him. Other than that, I don't have a particular sound in mind, but there's always a distinct feeling of "this isn't a single sound, it's a chord/harmony" to him.

Also both of mine have their own perms on their journals if you'd be so kind [fingerguns]