Which operating system do you use as a server? Make sure that you have a proper inputs/outputs configuration for both client and server. On a client your input/output should be related to the hardware. If it was working before, probably some settings were changed. Not necessarily NoMachine’s. Did you change any of them related to sound before you noticed the problem with audio streaming?
How does it with other browsers? Does it behave in the same way?
Did you try to follow this instruction?
(…) try to uncheck nxnode app – reboot machine – check nxnode again. If this also did not help, you can add whole NoMachine application to Accessibility.