The default is always the first available stream. So there is no way of specifying it during the scanning stage. You can not mass-edit DVB channels either. The way I recommend is to do a right-click, Audio Programs, choose correct audio, and then Right-click, Audio Programs, Use current audio as default, while playing a channel. Alternatively you need to open Manage Channels and edit each channel by setting the correct audio PID for each.
That said, I will take a look at the scanning code and see if we get any info about the type of audio we are getting in each stream. If we can, we might try avoiding choosing the "audio description for visually impaired" type.