The current pre-release of the client ("pioneer" in the version) is only compatible to itself. So you can only play with other testers. Please be aware!
Usually, surround sound causes problems with Supreme Commander.
Just enforce stereo sound in your windows settings or sound software.
Обычно объемный звук вызывает проблемы с Supreme Commander.
Просто включите стереозвук в настройках windows или звуковой программы.