udpsink does not work with IPv4 destination addresses on MacOSX. Tried on both 10.7 and 10.8. All systems were multihomed and IPv6 configured to "automatic", in case that matters.

The easiest way to see the problem is as follows:
   $ gst-launch-1.0 -v audiotestsrc  ! mulawenc ! rtppcmupay  ! udpsink host="localhost" port="13002"
works fine.
   $ gst-launch-1.0 -v audiotestsrc  ! mulawenc ! rtppcmupay  ! udpsink host="127.0.0.1" port="13002"
fails, with errors
   WARNING: from element /GstPipeline:pipeline0/GstUDPSink:udpsink0: Error sending UDP packet
   Additional debug info:
   gstmultiudpsink.c(545): gst_multiudpsink_render (): /GstPipeline:pipeline0/GstUDPSink:udpsink0:
   Reason: Error sending message: Invalid argument

The problem turns out to be (after an incredible amount of debugging:-) that gstmultiudpsink opens an IPv6 socket, and then does a sendmsg() to an IPv4 address.

Apparently this works on Linux, but not on BSD systems, see for example <http://stackoverflow.com/questions/8087478/using-sendto-on-a-ipv6-socket-to-send-to-a-ipv4-address>.

The workaround is to use force-ipv4=true on udpsink, obviously. However, I think that at the very least a better error message would be in place. Even better would be converting the IPv4 address to its ::ffff: form.