Hi,
I have filmed our band at a concert with a small handy recorder (zoom q3) which doesn't have good video, but quite good audio. I would now like to normalize the audio without reencoding the video (which is in 640x480). Every time I try, I get an output with black bars around the video. The video also seems to be smaller than the original.

Can someone provide me with help on how to set the rendering etc. right? Thank you very much. There are so many options that I don't really know what to do...

Maybe will be easier to split audio, then normalize audio in Audacity, and then sync back with the film? I usually use separate Zoom recorder for my films, and then syncing them. It takes bit time, but with some experience is not too bad.

All you can really do is manual audio gain adjustment. Or, depending upon your version, you can apply the ladspa filters for dynamic range compression and limiting.