OK, this is not a FIX you can apply yourself.... This needs to be updated with the launcher itself.
But its the reason why its broke (as far as I can tell)
Technic Launcher FORCES the 2.9.0 version of the Java Light Weight Game Loader (jlwgl.dll) and all its associated files....
It force overwrites it every time it launches the game. (I went to an extent to try to prevent overwrites, and if I got it not to overwrite it simply halted game launches and wouldn't start) It downloads and forces this 2.9.0 version to vanilla minecraft, all modpacks, tekkit, voltz, the big dig... everything.
2.9.0 was a junk version, it didn't work on a large handful of systems for the audio, its why we don't have working sounds!
Verify for yourself! Go download the real vanilla minecraft 1.7.4 from mojang, use their launcher, and then after it loads and audio DOES work, check the file versions of the jlwgl in the folders there, they are running 2.9.1
Can we PHUuuuuuLEEEEEeeeeeaaaase get the technic launcher to run 2.9.1 instead of 2.9.0 !!?!!?
it'll make all of us no-sound whiners really happy.
It changes absolutely nothing that might break the game as far as I know... However I do know that it fixes the audio.