Mal has heard Inara laugh genuinely before, generally when Shepherd Book would tell his unfailingly hilarious monastic tales-- but the sound is so rare that it's all the more pleasing to hear, especially when he's the cause of it.

"While I object strongly to the term 'pirate', I take your point." He chuckles, privately delighted to hear her say she feels at home on his ship, though he won't acknowledge it out loud.

"...and I don't doubt it. Though presumably there was something beneath the layers. Ridiculously complicated undergarments, for example."

Well they are discussing Companions, after all. It wouldn't be right if he didn't make some kind of risqué comment.