AFAIK you can only convert the AC3 5.1 audio in Nero Vision Express to AC3 2.0. To use 5 channel audio you need the multi-channel audio plugin which costs extra from what I've seen on forums and the Nero website.
There is one thing you could try and that's once you've imported the .avi file into Nero Vision Express, you can then double-click on the video that's been imported and then right-click on the video track and select Properties. A window should open with an audio-level control. The default level is 100% and you can move the slider to 200%. It doesn't make a great difference but it's better than the default for sure.
Edit: I'm also trying a free program DivX to DVD Converter at the moment. It's keeping the AC3 5.1 soundtrack, so I'll see how that goes.