It's been a while since I uploaded....
Anyway, yes, it's custom music that was inputted into the ISO (but I did legitimately buy the game from GameStop, hence the KH Outfit). My Custom Soundtrack (for anyone who cares about putting custom music....) is as follows:

FF1 - Battle = Destiny's Force (KH1)
FF1 - Dungeon = Holy Orders "Be Just or be Dead" (Guilty Gear, Remixed by SSH)
FF1 - Chaos Shrine = 13th Struggle (Kingdom Hearts 2)
FF1 - Town = Xepher (DDR SuperNova, Looped by me.)
FF1 - Mt. Gulg = Rage Awakened (Kingdom Hearts: Birth by Sleep)

FF2 - Battle Scene 1 = Battle Scene A (shAde)
FF2 - Battle Scene 2 = Scythe of Petals (Kingdom Hearts RE:Chain of Memories)
FF2 - Castle Pandemonium = Graceful Assassin (Kingdom Hearts RE:Chain of Memories)
FF2 - Rebel Army = Lord of the Caslte (Kingdom Hearts RE:Chain of Memories)
FF2 - The Imperial Army = Everlasting Crystal (Fanmade album of "Final Fantasy versus XIII" made entirely by youtube user "KayinOne")

FF3 - Battle 1 = Saturos Battle (Golden Sun, Remixed by Newgrounds user "Ivan-Ether")
FF3 - Battle 2 = Thanatos (Soul Calibur IV)
FF3 - The last battle = Tower of Death (Brave Fencer Musashi)
FF3 - The Crystal Tower = Pompeii (ES Posthmus)
FF3 - Let me know the truth = Tenrai II (Final Fantasy VII: Advent Children/Complete, Lopped by me.)

FF4 - Battle 1 (Retained, Plus I love how it was remade)
FF4 - Battle 2 = Caught - Cornered! (Phoenix Wright: Trials and Tribulations)
FF4 - Battle with the Four Fiends = Father and Son (Metal Gear Solid 2)
FF4 - Red Wings (remained)
FF4 - Theme of Love = Rival Battle Remix 2 (Pokemon, uploaded by youtube user "PokeRemixStudio") Besides, why would Square put the "Theme of Love" as a battle theme?????

FF5 - Battle 1 = Power Rangers (Remixed by "PowerGlove")
FF5 - Clash on the Big Bridge = BB Revenge (Remixed by "AKHT")
FF5 - The Final Battle = BB Evolution (Remixed by "AKHT")
FF5 - The Decisive battle = Chaotic Battlefield (Crisis Core)
FF5 - Home, Sweet Home = Deep Drive (Kingdom Hearts 2)

FF6 - Battle = The more I try (Tales of Symphonia: Dawn of the New World/Knight of Ratatosk)
FF6 - The Decisive battle = Decisive Battle (Remixed by SSH)
FF6 - Battle to the Death = Dancing Mad ~Tier 4~ (Remixed by "shAde")
FF6 - Dancing Mad = J-E-N-O-V-A (Final Fantasy VII)
FF6 - Searching for Friends = The Other Promise (Kingdom Hearts 2: Final Mix+)

FF7 - Fight on! = The Summoned Battle (Crisis Core)
FF7 - J-E-N-O-V-A (Retained)
FF7 - One-Winged Angel = The World's enemy (Crisis Core)
FF7 - Opening~Bombing Mission = Jenova Absolute (Final Fantasy VII Remastered by FinalFanTim)
FF7 - Let the Battles begin (Retained)
FF8 - Don't Be Afraid = Original FF8 Version (Final Fantasy VIII)
FF8 - Force Your Way (Retained)
FF8 - The Extreme (Retained)
FF8 - The Man with the Machine Gun = Liberi Fatali (Final Fantasy VIII, Looped by me.)
FF8 - Premonition (Retained)
FF9 - Battle 1 = Cornered! (Phoenix Wright)
FF9 - Battle 2 = Super Public Enemy (Devil May Cry)
FF9 - The Final Battle = Vergil Battle 3 (Devil May Cry 3)
FF9 - The Darkness of Eternity = Frost Dragon (Brave Fencer Musashi)
FF9 - Not Alone = Vergil Battle 1 (Devil May Cry 3)
FF10 - Battle Theme (remained)
FF10 - Otherworld (remained)
FF10 - Summoned Beast Battle = Decisive Battle (Final Fantasy X)
FF10 - Fight with Seymour = Summoned Beast Battle (Final Fantasy X)
FF10 - A Fleeting Dream (Retained)
FF11 - Battle in the Dungeon #2 = The Conflict (Crisis Core)
FF11 A Realm of Emptiness = FF6 Decisive Battle ver. 2 (SSH)
FF11 - Awakening = The Landing (Final Fantasy VIII)
FF11 - Iron Colossus = Final Battle against Xehanort (Kingdom Hearts: Birth by Sleep)
FF12 - Boss Battle = Fighting of the Spirit (Tales of Phantasia, Remixed by "SSH")
FF12 - Esper Battle = Treachery (Bleach, Yeah that's right. BLEACH)
FF12 - Fight to the Death = Sword Fight (Brave Fencer Musashi)
FF12 - The Battle for Freedom = The Stars Protection (Crisis Core)
FF13 - Blinded by Light (Retained)
FF13 - Saber's Edge (Retained)
FF13 - Nascent Requiem = Desperate Struggle (Final Fantasy XIII)

Cosmos = Mysterious Figure (Kingdom Hearts: Birth by Sleep)

Chaos: Last Battle 1 = A fight to the Death (Kingdom Hearts 2)
Chaos: Last Battle 2 = Darkness of the Unknown vs. Xemnas' Airship/Dragon thing (Kingdom Hearts 2)
Chaos: Last Battle 3 = Darkness of the Unknown vs. Xemnas [Final] (Kingdom Hearts 2)
Dissidia (remained)
Cantata Mortis & God in Fire (Retained)
...
https://www.youtube.com/watch?v=i-TbWHO2qAQ