This has nothing to do with the sound volume of helicopters. Speech is louder in third person then in first.
Speech is far too quiet in first person mode inside helicopters and too loud when in third person. Proper bug. If this exists in another ticket let me know and I'll close it!
To replicate, place the player's unit in the cargo of a helicopter in the editor and play a custom sound, or use KBtell, or use say3D etc.
You'll notice that the sound is barely audible, yet perfectly clear in third person.
Obliviously - this is wrong, as a person speaking inside a helicopter would be louder inside than outside.