Hi
Lets start with the simple things:
audio level in client is a OSC-problem
NDI:
in server 2.08 you use an older version of NDI called iVGA AirSend. That version does not support audio to NDI version 4.0 or higher. The fix is in this topic. The server 2.3 uses a real NDI consumer so you have to put NDI as a consumer in your config-file.
About the real problem so you dont say in which outputs the audio is crackeling, system audio, embeded SDI-audio in decklink, analog audio from Decklink or what.