Elmo,
Don't see any technological barrier to equipping a UAV to dispense and "listen" to sonobuoys.
'Listening' to sonobuoys involves rather more than you'd think - you get a lot of different sounds that you then need to analyse/identify, at which point IF one of the sounds is the guy you are looking for you then need to decide target speed, depth, heading, position from what you hear - this involves comparing info from several buoys usually, and whilst aided by a sound knowledge of geometry there's still a touch of an art to it... basically several people working out possible combinations that make sense, whilst rejecting a number of combinations that would match most of the data but don't make sense.
Then there's the number of sonobuoys required - on two consecutive sorties I flew on we dropped 99 buoys each day, it'd be one hell of a UAV that could get airborned with that load aboard, never mind the acoustic processing equipment etc as well.
No, there's no technical barrier to dropping the odd buoy and recieving what it transmits - but you won't find any submarines by doing that.
Dave