I believe it's bugged, the 1.9.json file shows names such as random.orb, random.explode, etc. but they don't work. The only sounds I've been able to play (Not that I've tried a lot) are entity.wither.spawn and ambient.cave (Which functions the same as 1.8 ambient.cave.cave). So yeah, I think something's bugged. But regardless I can't find documentation on the 1.9 play sound changes anywhere.
Edit after some more searching I found a 1.9 bug report with an attached file with the proper names: https://bugs.mojang.com/secure/attachment/102693/sounds.json
random.explode has become entity.generic.explode.