*UPDATE* - As pointed out by @bighap83, it's now showing officially supported on Z10 spec page.
When the BlackBerry PlayBook initially launched there was a good many folks wondering about video Playback support. As it stands, the BlackBerry PlayBook can play a lot of video formats out of the box but there is a lack of AC3 audio support. The AC3 audio codec is often used in MKV and AVI files so, that meant PlayBook users had to convert the audio before watching their videos. If you're a new BlackBerry Z10 owner, that process is no longer a problem as AC3 audio is up and working on BlackBerry 10 but, it does seem as though it's not 'officially' supported.
After some testing of video files, we've been able to confirm there is indeed support for AC3 audio within BlackBerry 10, MKV and AVI video files encoded using that as their audio source. Now, keep in mind there is a lot of files out there depending on where you get them from that come in many formats so alas, there is always the chance you'll come across some that just simply won't. That said though, in our testing 8 out of 10 files worked just fine, which is a good ratio.
Learn more about supported video/audio formats on BlackBerry 10