In Metal Gear Solid 5: The Phantom Pain, when you take-off or land, you can set a default song to play. How do you do that?
There are a couple of things involved in having your helicopter play music out loud.
Since the SPEAKER development is an upgrade, denoted by the giant plus sign on the right side of its icon, you do not need to equip it explicitly. Once development is complete, the helicopter automatically has the loudspeaker built into it.
Now you can now assign music in the iDroid > Missions > Cassette Tapes screen:
Once you have the desired song highlighted, the button to press is Square (on the PS4) for Set Helicopter Music . Your platform will show the appropriate key bind at the bottom there.
I have heard that players on PC can import custom music of their own, but I haven't had any direct experience with that process myself.