Ok, I downloaded the campaign, extracted the files to have a look. Now I don't have DBW, only HSFX7 and VPmod both in a 412 environment, so I'm unaware of the format DBW is happy with playing those mission files, but they are not in a format the same as I am used to seeing in my games. So the first thing I did was copy the campaign folder (Enterprise_Gold) to IL-2's missions folder so I could access the files in FMB.
Started my HSFX7 game, enter FMB, select Load, select Enterprise Gold and load-up mission one. I then saved the mission which should save it in the format relative to my game environment. Clicked play and had a look around. Let auto pilot do the work for takeoff and checked for flyby.
All is working fine. Again save the mission, just for safe keeping and exit the game to view the log for any errors and there were none.
Restarted the game and repeat the process but now load-up mission 2. This time I had to make a couple of changes to 2 Japanese flights that were air start but had a 0 height at way-point one and 2 so made an adjustment, saved the game, played it and again all was working fine.
This is, I believe, what you will have to do to every mis file in your DBW game for this campaign to work properly, and that's provided that my flyby mod is working in your game as you say.
My advice to you before starting this would be to load-up a mission file in FMB you know is causing the no flyby issue, and do as above and test it. If you have success then you know what you have to do if you are passionate in playing this campaign. If you have no change and the flyby is still not audible then I will see about locating the smaller wav files for flyby.
Good luck!
Cheers DaveB
NB: When I extracted the campaign it's file structure had it ending up in a NN folder. As I don't have that folder I put it in UN.