I am sure most of the CM users have had this issue with their bluetooth headsets not working for VoIP calls on apps like Viber, Skype etc.
Sadly the same issue occurs with me too. There are no issues when i use a sammy stock based roms however this happens only when using a CM based rom.
I am using a Sony SBH 20 and also have a Samsung HS3000 bluetooth headsets and have tried almost all the roms including the official CM 11 nightlies, CM 11 by MarkOX, SlimKat, Carbon, PAC so on. none seem to work with bluetooth and viber.
However, the problem was not existing when i was using a Samsung Galaxy Grand (GT - i9082) with pawitp's CM 11 and Xenons Carbon rom (Both based on CM 11 sources) and bluetooth with viber or normal calls never had an issue at all.
Wondering if this is something to do with the Audio part of the roms to be causing compatibility issues with bluetooth because the same issue appeared resolved for roms cooked for an other device.
Pleading for a fix or a suggestion if any audio libs can be copied to the system folder of CM 11 from sammys stock to fix this issue (if anyone has tried it).