In my case, the reason was that I had deactivated my sound-card in Windows Hardware Management. (On another PC I did not even have the sound-card driver installed, as I never used sound on it: it did not have speakers...) Once I installed a driver for my sound-card and activated it in Windows Hardware Management, multiplayer was working fine...
(Took me over a year to find this out by coincidence...)
PS: can a mod please move this to the "Technical Support" forum? I think it better fits there.