The ia32-libs that ships with Ubuntu doesn't contain 32 bit libraries for all the useful codecs of gstreamer. There's enough to build Wine, but not enough to actually show a video.
I've fixed this in the ia32-libs on the Wine PPA by adding _73_ new libraries to it. This is something you'll want even if you're not using the wine packages and just doing Wine development.
Thankfully, ia32-libs should be finally obsolete by 11.04 with multiarch, but in the meantime we can use the PPA.
I'll work on porting the ia32-libs changes to 10.04 and 9.10, but it's less of a priority. On those we have other problems (such as old GCC versions causing problems).
Thanks, Scott Ritchie