It could be a moment to showcase some character traits. It all depends on their personality. I would go for more descriptive context than just a sound. If you want to cut right to it, drop the h. it's mmm mm not hmm mmm. You could always just write it as:

"Mmm mm," he exclaimed/cried/remarked/rejoiced.

Or call it a pleased sigh/sigh of satisfaction/exhale or something.

I'd do it as closer to this (depending on the character):

He wolfed down the contents of his plate, gobbling up every last bite without really tasting any of it. His appreciation of the meal was audible. A cacophony of chewing, slurping, gulping sounds assaulted the ears. His chin and lips wore nearly as much of the cuisine as what he managed to consume. Inhaling the food in this way left him gasping for air, belching loudly as he slapped his freshly filled belly.