CM11 VoIP internal microphone
The same here. Yes it is bug in CM11 on my Samsung Galaxy S4 9505 as well. Internal microphone don't working with VoIP applications. I already tested with Skype, Viber and Hangout.
Alternative solution is to connect headphones with microphone.
This makes mobile bit useless if you want to make abroad calls via VoIP apps. I really appreciate if CM team can fix it on one of next coming nightly update.