The below is taken directly from the Nokia Belle FAQ:
After the update to Nokia Belle I can't play some .flv flash files or .mkv video files anymore. What should I do?
We are currently investigating the issue and will share a solution as soon as possible.
This is a known issue and our tech team are working on this issue. Please be patient as a fix will be available as soon as possible.
Hi Nokia Team,
I have been looking for a solution for more than a month. Belle was updated a month ago and still no updates from nokia regarding this issue. Also, with my Nokia 603 i couldnt play divx and Xvid files. There are some video files which play without audio. I want an answer when it is going to be rectified.
Strangely, I'm able to play FLV's still on my N8 running Belle. I've tried various FLV's of different qualities and all play without issue.
Those who are still having problems, you can convert your FLV's to MP4's using a free video converter. Also, when downloading videos from the internet look for an MP4 version instead of an FLV. Many popular YouTube downloaders give the option to download as an MP4.