Comment 34 for bug 1104373

Same issue - playback has sound but no video - just see underlying desktop wallpaper

Useful information:

lspci | grep -i vga
01:00.0 VGA compatible controller: NVIDIA Corporation NV44 [GeForce 7100 GS] (rev a1)

 grep -i nvidia /var/log/Xorg.0.log | head
[ 16.732] (II) Module glx: vendor="NVIDIA Corporation"
[ 16.732] (II) NVIDIA GLX Module 304.88 Wed Mar 27 14:46:57 PDT 2013
[ 16.732] (==) Matched nvidia as autoconfigured driver 0
[ 16.732] (II) LoadModule: "nvidia"
[ 16.732] (II) Loading /usr/lib/x86_64-linux-gnu/xorg/extra-modules/nvidia_drv.so
[ 16.733] (II) Module nvidia: vendor="NVIDIA Corporation"
[ 16.772] (II) NVIDIA dlloader X Driver 304.88 Wed Mar 27 14:28:14 PDT 2013
[ 16.772] (II) NVIDIA Unified Driver for all Supported NVIDIA GPUs
[ 16.772] (II) NOUVEAU driver for NVIDIA chipset families :
[ 16.798] (II) NVIDIA(0): Creating default Display subsection in Screen section

Fresh install of ubuntu 13.04 amd64, albeit minimal install with --no-install-recommends