Find the zoo on Mother Base

Customizing weapons is not very hard, you just need to know where to look.

First things first: You need to have completed a mission where you

Extract a Legendary Gunsmith

It's a Side Op, and is labeled as above. He'll become a part of your staff and you'll be able to customize weapons.

You're now done with the above, so how do you go about the actual "customization"?

• First you need to be in the ACC (Aerial Command Center), you get there by just calling your chopper for a transport.

• Next, you open your iDroid and go to the Mother Base tab and choose Customize.

And there you go, you should now have several options for customization.

There are a couple of things involved in having your helicopter play music out loud.

First, you must develop a loudspeaker for the helicopter itself, by going to iDroid > Mother Base > Development > Helicopter:

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.