Zandronum stopped working well when I upgraded my desktop computer distro to Slackware 14 from Debian Squeeze. On some maps it would die with error:
execvp failed zandronum: Fatal IO error 11 (Resource temporarily unavailable) on X server :0.
Big thanks to Dusk from #zatest on Zandronum IRC for helping to debug it. Comes out the error occurs only when Timidity is selected as MIDI output but no Timidity is installed. On Debian I used Timidity and during upgrade I just copied my whole home directory, including the old configuration file of Zandronum. Switching to FMOD as the MIDI output resolves the issue. Installing Timidity would probably solve it too.