Change:
export PATH=$PATH:native -java $CLIENTARGS -classpath "lib/felix.jar:sc-bundles/sc-launcher.jar:sc-bundles/util.jar:lib/" -Djava.library.path=native -Dfelix.config.properties=file:./lib/felix.client.run.properties -Djava.util.logging.config.file=lib/logging.properties net.java.sip.communicator.launcher.SIPCommunicator <del> +exec java $CLIENTARGS -classpath "lib/felix.jar:sc-bundles/sc-launcher.jar:sc-bundles/util.jar:lib/" -Djava.library.path=$NATIVELIBS -Dfelix.config.properties=file:lib/felix.client.run.properties -Djava.util.logging.config.file=lib/logging.properties net.java.sip.communicator.launcher.SIPCommunicator </del> <ins> +exec @JAVA@ $CLIENTARGS -classpath "lib/felix.jar:sc-bundles/sc-launcher.jar:sc-bundles/util.jar:lib/" -Djava.library.path=$NATIVELIBS -Dfelix.config.properties=file:lib/felix.client.run.properties -Djava.util.logging.config.file=lib/logging.properties net.java.sip.communicator.launcher.SIPCommunicator </ins>

Discussion:
Using jitsi-2.4.4997 on Ubuntu Precise I get the following: 14:43:06.925 SEVERE: [13] () Failed to register custom Renderer nderer with JMF. audioSystem at &lt;init() at &lt;init() at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) at sun.reflect.NativeConstructorAccessorImpl.newInstance() at sun.reflect.DelegatingConstructorAccessorImpl.newInstance() at () at () at () at &lt;init() at &lt;init() at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) at sun.reflect.NativeConstructorAccessorImpl.newInstance() at sun.reflect.DelegatingConstructorAccessorImpl.newInstance() at () at () at () at () at () at () at () at () at () at () at () at () at () 14:43:06.929 SEVERE: [13] () Failed to register custom Renderer with JMF. /nix/store/4lmjhyh1rbn90w1klmmms66f5jf857d5-jitsi-2.4.4997/lib/native/linux- cannot open shared object file: No such file or directory at $(Native Method) at () at () at () at () at () at &lt;clinit() at (Native Method) at () at () at &lt;init() at &lt;init() at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) at sun.reflect.NativeConstructorAccessorImpl.newInstance() at sun.reflect.DelegatingConstructorAccessorImpl.newInstance() at () at () at () at () at () at () at () at () at () at () at () at () at () Fontconfig warning: line 26: empty font directory name ignored 14:43:07.192 SEVERE: [13] impl.osdependent.SystemTray.getDefaultDesktop().62 Failed to initialize the implementation.

at $AWTDesktopPeer.&lt;init() at () at &lt;init() at () at () at () at () at () at () at () Caused by: at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at () at () at () at $AWTDesktopPeer.&lt;init() ... 9 more Caused by: Desktop API is not supported on the current platform at () ... 14 more 14:43:07.193 SEVERE: [13] impl.osdependent.SystemTray.getDefaultDesktop().70 Failed to initialize the implementation. Failed to initialize the implementation. at () at &lt;init() at () at () at () at () at () at () at () 14:43:07.560 SEVERE: [38] util.UtilActivator.uncaughtException().108 An uncaught exception occurred in thread=Thread[AWT-EventQueue-0,6,main] and message was: null at () at $2.run() at java.security.AccessController.doPrivileged(Native Method) at &lt;init() at &lt;init() at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) at sun.reflect.NativeConstructorAccessorImpl.newInstance() at sun.reflect.DelegatingConstructorAccessorImpl.newInstance() at () at () at $1.run() at java.security.AccessController.doPrivileged(Native Method) at () at () at () at () at () at () at () at &lt;init() at &lt;init() at &lt;init() at () at $1.run() at () at () at $200() at $3.run() at $3.run() at java.security.AccessController.doPrivileged(Native Method) at java.security.ProtectionDomain$1.doIntersectionPrivilege() at () at () at () at () at () at () at () ^ Listener is null at () at () at () at () at () at () at () at () at () Listener is null at () at () at () at () at () at () at () at () at ()
On NixOS 15.09 I get similar error messages. Jitsi's GUI does appear (after applying ), and my webcam is recognized, but no audio system is detected, despite having Pulseaudio enabled. The audio options are or . what's the status of this for you?
Was this fixed by ?

had two errors that I didn't: I haven't run Nix outside of NixOS yet, so I can't say whether it's fixed right now, but if it's still present then it's a problem interacting with that environment, and fonts not loading seems like a good reason for a GUI not to appear. ( fixed the errors I saw except for the system tray ones.)
This fontconfig warning is harmless IIRC.
I think this issue is independent of my fix after all. I installed Nix and Jitsi in a fresh 12.04 VM and Jitsi opened up fine on nixpkgs-stable (nixpkgs-16.03pre68896.), which is before that got merged. Then again, Jitsi did start for me, so maybe something else was fixed in the meantime.
(triage) do these problems still appear?
Last time I tried ^, I got some of the same warnings, but I was never able to reproduce. I think this is obsolete now.
Works for me, too.