I don't know why but for some reason Minecraft doesn't load the sound system sometimes. I don't know what exactly triggers it, but sometimes (as in perhaps a 1/10 chance every startup) it doesn't load.
Attached I have the log of the launcher. It rants on about not being able to find the library, but if that were the case, then how could it load before?
Environment
Windows 8, 2 core CPU @ 2.16 GHz (laptop), self-updating Java
I've had this happen a little less frequently on 1.9.4