You should only need to add some stuff to the configure command line. To tell it to compile in netaudio support:
--with-sound=both
(Or `--with-sound=nas' if you don't want native sound support for some reason.) To tell it where to find the netaudio includes and libraries:
--site-libraries=WHATEVER --site-includes=WHATEVER
Then (fingers crossed) it should compile and it will use netaudio if you have a server running corresponding to the X server. The netaudio server has to be there when XEmacs starts. If the netaudio server goes away and another is run, XEmacs should cope (fingers crossed, error handling in netaudio isn't perfect).
BTW, netaudio has been renamed as it has a name clash with something else, so if you see references to NAS or Network Audio System, it's the same thing. It also might be found at ftp.x.org:/contrib/audio/nas/ ftp.ncd.com:/pub/ncd/technology/src/nas/