esdlib.c:

/**
 * esd_play_stream_fallback: as esd_play_stream() but connect directly if no ESD
...
 * Return Value: -1 on error, else a socket number set up so that
 * any data sent to the socket will be played, either by an ESD or by an.
 */
int esd_play_stream_fallback( esd_format_t format, int rate, 
			      const char *host, const char *name )
{
    int socket_out;
...
    socket_out = esd_audio_open();
...
    return socket_out;
}

audio_alsa09.c: (ALSA-1.0 backend is enabled)

int esd_audio_open(void)
{
...
	alsa_playback_handle = initAlsa(dev, format, channels, esd_audio_rate,
SND_PCM_STREAM_PLAYBACK);
...
	esd_audio_fd = 0;
...
	return 0;
}

The problem with the constant zero "descriptor" is that it doesn't work in some
essential cases (wsoundserver/wsoundplay are known to break).