Yeah, they should really purchase some sound packs so everything is legit. I also totally agree with the sound radar.
In theory, as it is now, I can mod my IL-2 46 sounds to say "RUBBERBABYBUGGYBUMPERS" at max volume when an enemy is firing guns.
Or I could simply record myself saying "100 meters" "200 meters" and then add these tracks to every engine in the game and set the distance to when I hear these sounds to the specified distances. That way I would know the distance to the surrounding aircraft....
I hope sound mods will be banned from the vanilla game, and only allowed in modded servers where EVERYONE is required to have the same sounds.
IRL it should be darn impossible to hear somebody else's engine from internal cockpit view due to the insane volume of your own engine. So maybe multiplayer is better balanced by NOT having the sounds of other aircraft being hearable from inside the cockpit.
Either way, I think this debate ends like KG alpha said: Copyright problems.
We'll have to wait until the devs legally aquire sounds, or design them from scratch! (E.g record Oleg's Lada at all RPM's and overrev it to create damage samples, then HEAVILY modify them to sound like DB600's

)