Well, what I did for the codecs is went to
www.videolan.org/vlc and downloaded that nice little free app: you won't need any additional codecs with that player. You'll only need to configure it as your default player.
As for the preview function, you need to go to the LW preferences/Options.
From there you need to click on the Player tab from the list on the left and then tick the box and click Apply (if it is already ticked, try unticking it, Apply, then re-ticking it)