On my D630 and Dell D-Port Replicator running OEL5U2 I cannot get any sound from the jack on the port replicator.
Thanks to http://ubuntuforums.org/showthread.php?p=2218245#post2218245 below is the way to get it running:
In the volume control, under the switches tab, enable IEC958. If the
switch isn’t there, unhide it from within the preferences menu.
You may now plug your external speakers into the headphone jack of the dock.