The PC version of Metal Gear Solid V: The Phantom Pain supports custom soundtrack files which can be gayed on the in-game Walkman, or over the in-game helicopter's speakers during attack runs and landing/takeoff. It should be noted that (as far as I know) only .MP3 format files are supported. I have tried .M4A and .FLAC, neither are recognized.

To play custom music in MGS5, you must place your music files in the "CustomSoundtrack" folder located in the installation directory for the game. After this, you can access said music files in-game by following these steps:

1) Open your iDroid
2) Go to "Missions" tab
3) Select "Cassette Tapes"
4) Select the "Music" tab
5) Select "Musical Content" from the list
6) From here you can either play the song or set it as your helicotper music

For me, the problem was that trying to play a custom music track would cause the game to skip that track and go on to the next one, where it would repeat the same thing again. Normal music worked fine, so I was confused. Visually it appeared that the "Now Playing" icon was quickly scrolling down the list. Long story short, something was causing the audio for each custom music track to skip from the beginning to the end. Applying a custom music track to the helicopter would result in no music being audible.

I downloaded the basic version of the K-Lite Codec Pack (link[www.codecguide.com]). After installation, I opened the Codec Tweak Tool and did this:

1) Under "General", click Fixes
2) Select all boxes
3) Click "Apply & Close"
4) Proceed through the following dialogue boxes, fixing anything that comes up
5) Return to the main screen of the application
6) Under "Codec and Filter Management", I activated all audio codecs (with the exception of the ones listed under "Broken") under all categories. Click apply and close after each one.
7) Close the Codec Tweak Tool and launch MGS5
8) Congrats! You've used the power of codecs to fix your game!

Let me know if this works for you. Hopefully this is an end-all solution. In all honesty, activating all the codecs probably isn't the solution. It was probably just one (or lack of therof) that was causing the issue, but more of an "if it aint' broke dont' fix it" type of guy, so let me know what you come up with. Good luck!