kimINSOUCIANT  is an adjective of French origin which is variously used to define someone’s behaviour as  ‘carefree’, ‘unconcerned’, ‘light-hearted’, ‘nonchalant’ or ‘indifferent’.

Michael Azerrad, witing in the very excellent ‘Our Band Could Be Your Life’ described the voice of Sonic Youth‘s Kim Gordon as “a sort of insouciant holler, like a kid calling to her friends about something great she’d found but trying not to seem too excited about it”.

On the strength of this I’ve just added it to my Wordnik list of words describing singing voices.