Specifically, in my setup, I'm playing Alpha 1.0.17_04 and am trying to play with sound. I've tried everything with the beta launcher to get sound in game. I decided to try the latest non-beta launcher (as I had previously been able to get sounds to play, even on older versions). First try, sounds worked. I tested 4 different situations once more.

 * latest non-beta launcher, with jvm argument
 * latest non-beta launcher, no changed jvm arguments
 * latest beta launcher, no changed jvm arguments
 * latest beta launcher, with jvm argument

The ONLY one which worked and I got sound was my first test (non-beta, with jvm argument)

jvm argument used: "-Dhttp.proxyHost=retrocraft.net -Dhttp.proxyPort=80"