The actual problem is, that the sound channels get overlaoded. Warband only has 100 sound channels and when you have 200 guys walking, you already have to many sound channels playing.
We tried to fix this problem with getting priorities for specials sounds. For example, footsteps should not be played if someone is shooting.
Since the new version it somehow seems a bit bugged, but we are working on it. Strangely, I could fix the problem for me, with just making the ingame sound abit quiter.
It is not related to it at all, and makes no sense, but you could try this aswell. Other than that I am really sorry for the problem, and we are looking into fixing it.