I'm afraid it would require fixes in kmplayer code. It just wasn't created with multiple audio tracks in mind.

You can try requesting feature enhancement in OMP thread - it already have "zoom", so it could have "stretch", probably (especially, that backend is the same as in your kmplayer case). I think it wasn't implemented as no one, ever, considered it as useful feature

Other than that, there is only re-encoding. Either to get rid of additional audio tracks and leave video stream untouched, or to leave audios as they are, but stretch videos to 800x480 at encoding time.

/Estel