Dear guys,
I started to play 1 month ago, but I was unable to play 2.5.6 open beta becuase I was unable to hear any sound from the game itself (the same thing was not happening in 2.5.5.41962). After several trials, I discovered that lower the audio from 48.000 Hz to 44.100 Hz in the audio proprieties - advanced settings, could solve your problem. At least it solved it for me.
Hope it was helpful.