As another test of speed, I have a GA that takes a few days to complete. Due to the lengthy run time, a difference in performance between VB6 and .NET would be highly noticeable. If anything, .NET was slightly faster, probably due to better design, since GAs are highly suited to OO design. However, the difference in speed was only a few hours, at best.

If you are seeing a significantly slower performance for audio processing, there is probably a reason for that which is specific to that particular task. I would guess that VB6 was making use of a library that wasn't part of the language, in which case it is equally available in .NET.