1. demux ac3 from avi using vdubmod
2. fix ac3 using ac3fix
3. remux fixed ac3 using vdubmod

load original clip in vdubmod or nandub -> properties show ac3 is 5 chan
load fixed clip in vdubmod or nandub -> properties show ac3 is 6 chan
load original clip in gspot -> properties show ac3 is 5 chan
load fixed clip in gspot -> properties show ac3 is 5 chan

I will guess the problem is not with gspot, since I can't imagine ac3fix would actually "create" a new channel.

Why is this a problem? Because vdub won't join two clips that it thinks have different numbers of channels.