Playing videos on VirtualBox Mac OS X guest on Ubuntu 13.04 as host I managed to install Mac OS X Moutain Lion as a VirtualBox guest OS on Ubuntu 13.04 as host.

I need to test a web app which plays HTML5 videos on Safari 6. Unfortunately, I can’t play any video on Moutain Lion (neither HTML5 one nor a .mp4 with Quicktime).

I guess it’s a codec or video card driver problem but I can't find a clue on how to fix this. Can anyone suggest an idea?
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.