[It takes him a second -Cloud just thought he was making sounds- but he becomes aware of the fact that Zack's humming along with him. So...it was a song Zack knew. Probably sang it in the lab, or while they were on the run.
With that realization, Cloud lets the song drift off towards the end, falling back into silence. The weight of the situation falls back down on his shoulders and he tenses to keep from shaking. He'd almost lost Zack, not to death but to the thing he'd actually wanted the most; his strength, his mako. And it was something Cloud would never have been able to protect him from; if John hadn't taken him, Zack probably would have eventually gone of his own free will. His fingers still in Zack's hair and he holds him a little tighter, locking his jaw.]
[Action]
With that realization, Cloud lets the song drift off towards the end, falling back into silence. The weight of the situation falls back down on his shoulders and he tenses to keep from shaking. He'd almost lost Zack, not to death but to the thing he'd actually wanted the most; his strength, his mako. And it was something Cloud would never have been able to protect him from; if John hadn't taken him, Zack probably would have eventually gone of his own free will. His fingers still in Zack's hair and he holds him a little tighter, locking his jaw.]
...Zack...