Audio may be missing when connecting to a Windows XP host with NoMachine 4.
This issue occurs when a non-recent version of Creative Sound Blaster driver is installed on the Windows machine.
We reproduced the problem with Creative Sound Blaster Audigy.
During disabling/enabling of NoMachine audio driver, a dll installed by Creative Sound Blaster driver is executed. This dll doesn't have valid certificate and the Windows system disables both the Sound Blaster and the NoMachine driver.
Updating Sound Blaster driver to version SBAX_PCDRV_LB_2_18_0017 or later should fix the problem.