After upgrading the NoMachine server to v9.0.188, audio is no longer available in web sessions. On Linux, this occurs when PulseAudio is installed on the system.

As a workaround, set in server.cfg:

AcceptedWebMethods  webrtc

or, if you want a failover in case the browser doesn't support webrtc:

AcceptedWebMethods  webrtc,mjpeg

Then connect a new web session.

Notes:
- order of values is relevant, webrtc must be the first one
- remove the prepending # from the key name, if present