For consistency this should be added to autovideosink too then. Note that this doesn't solve the problem in general, you are still not guaranteed that every sink implements that. They could still be a bin or a generic GstElement or whatever.

Also all these property proxies in autoaudiosink should probably first check if the child element actually has this property... and set the child property when it is created (which is missing in your patch, see the ts-offset handling in gst_auto_audio_sink_detect).

I agree that it would be nice to have better handling of missing properties. However, I just copied the code for ts-offset to provide the required functionality for my use case. 

The child property is set after child creation, similar to ts-offset. 
See attached patch, line 52ff:

@@ -336,6 +344,7 @@ gst_auto_audio_sink_detect (GstAutoAudioSink * sink)
     goto no_sink;
 
   g_object_set (G_OBJECT (esink), "ts-offset", sink->ts_offset, NULL);
+  g_object_set (G_OBJECT (esink), "sync", sink->sync, NULL);
 
   sink->kid = esink;
   /* Ensure the child is brought up to the right state to match the parent

Created attachment 253351 [details] [review]
Patch to add "sync" property to autovideosink

Add "sync" property to autovideosink

Oh sorry, for some reason I missed that in the original patch. Thanks for the autovideosink patch too :)

commit f7528d274b56f38a93461cb6f10bdf9775d52ba8
Author: Bernhard Miller <bernhard.miller@streamunlimited.com>
Date:   Wed Aug 28 10:51:32 2013 +0200

    autovideosink: add sync property
    
    https://bugzilla.gnome.org/show_bug.cgi?id=706955

commit 2fa68fce077a738821373ec8c16d90485ff4d697
Author: Bernhard Miller <bernhard.miller@streamunlimited.com>
Date:   Wed Aug 28 07:15:00 2013 +0200

    autoaudiosink: introduce sync property