I think you are on the right way.
Basically every item in the Images and Documents module has an URL, and URL with extension properties. You could use those to extract the item path and pass this variable to the media player, which will play the item. I am not really sure if your FLV items can be played directly, most of the times they need a player to get them streaming to the client.
On a side note, where did you insert this code? I could test this scenario, but I need to know which template you have modified. If it is possible to debug the code, please check whether there is something DataItem object.
All the best,
the Telerik team