I heard this issue come up on range vox yesterday, so I thought I'd post it here just so everyone is aware:
For those of you who use the German 30mm Mk 108 cannon in the 109K-4, 109G-14, 190A-8, Bf110G-2, Ta152, etc, I'm sure you've noted that the actual rate of fire does not match the default sound file. The default sound file is about half of the actual rate of fire, I do believe. Back in February of this year I put in a request in the sounds forum for someone to create a proper rate of fire sound file to use for the German Mk 108 30mm. Mr. "potsNpans" came through with a splendid file and it works perfectly. I believe it just sped up the current file to match the actual rate of fire. Now, when I fire the 30mm the diminishing round count matches the rate of fire (650 rds/min).

I just wanted everyone to be aware that a correct sound file is available for the German 30mm. Just download the file then put it directly in to the sounds folder. Easy.
http://bbs.hitechcreations.com/smf/index.php/topic,345284.0.html Also, FWIW, the Me410 has a pair of 30mm cannons as well. However, these cannons (Mk 103) fire the far flatter shooting 30x184mm cartridges, with a slightly slower rate of fire (380-420 rpm). potsNpans made a correct file for that too, the link is in the same thread.