CraterMA, TT area, I was driving the M4 76mm version (still missing a muzzle brake) while also updating another program (Photoshop through Creative Cloud not that it matters). When the update finished AHIII Beta minimized for just a short moment and I re-entered. The tread sounds, and the sounds of the sod tearing could no longer play. After a little driving I backed up and the sounds returned briefly, and then would only play at random times.
The Crater area seems to be non-optimal for frame rates.
Gun fire sometimes drops frame rates, though I think it is the playback of the reload sound of the maingun that is causing the largest drop in frames 2-4 fps.