(I'm posting this on all threads i can find because it was also very frustrating for me, with no solution whatsoever).
I had this same exact problem on my Galaxy S3. All my volume settings are at highest, and even when i press the volume up rocker during the call, it doesn't get louder. HERE'S THE SOLUTION THO (after countless hours of trying many, many different things)! Not sure if this works for other phones.
During the call, if you press the volume button, you should see the volume level bar with the "settings" icon next to it appear on your screen (look at the screen while the call is in progress). Press the "settings" button to open up the volume menu. If you're having low volume, you probably have all the volumes at their max. Put your finger on the volume slider and just move it lower then back higher, and you should notice that the volume goes higher than it was before. It's almost like it's stuck at what it thinks is the MAX level, and by sliding it back and forth, it's "unstucking" it, and allowing it to go higher to the true max level. Not sure why it does this, but after doing this, it should remain at the new higher volume for the next calls. I did notice that if you adjust the volume lower for anything else (say when you're watching a video, or turning it down because the ringer or notification sound is too loud), then it will get back to the low call volume and you have to slide it again to get it unstuck.
If anyone knows how to report this issue to the CM 11 coders, please forward...
Hope that helps!