Playing videos on VirtualBox Mac OS X guest on Ubuntu 13.04 as host
OS X uses QE/CI (Quartz Extreme/Core Image) for graphics acceleration. Unless you're using an actual Mac or are running a modified version of OS X directly on PC hardware, you won't get full acceleration. Furthermore, VirtualBox support for OS X is experimental and doesn't include Guest Additions. 

Looking around a bit, it seems some have had success with modified SVGA drivers in VMWare. It may get you better performance overall, but it still won't enable QE/CI (and thus no video playback), and doesn't work with VirtualBox anyway.