[AD] Allegro mixer in 4.4 Windows

[Peter Wang <novalazy@xxxxxxxxxx>]

I did some testing of the 4.4 branch in Windows today on my new machine.
The sound examples are absolutely scratchy and horrible, like a
multiplication has gone wrong and all the samples are clipping like mad.

When I set digi_card=DXA in allegro.cfg it sounds fine, i.e. it's only
when using our software mixer.  It works fine in Linux.

I will debug it, but I'm surprised it hasn't been reported before AFAIK.
Anyone encountered this before?

Peter