I use the method described above for my missiles, and the "object class" of these missiles is in most cases a missile, a bomb or a shell. So far the sound works well, while the maximum speed I did test is only around 900 m/s.
You may do a test, attach a camera to the your projectile (a _shell switchCamera "EXTERNAL" should be enough) and check how that sound is heard this way, because may be your problem is with the sound propagation system of ArmA.