You need to use the Unity Assets Viewer for that.
Open the resources.assets archive, search for the file "BtlEncountBgmMetaData.txt", export it, modify it and then re-import it.
Inside that file, there's a list of field IDs (so the Queen's Chamber in which you fight Beatrix is 1223) and for each field, a list of battles that can trigger in that field with the music ID of the battle ran. If a battle is missing there, the ambiant music doesn't stop when entering a battle.
So just find the field (1223), the battle (73) and put the correct music ID for it (117, "The Wavering Blade").
Here's a list of music IDs. I don't think that you can actually see it in Hades Workshop (only in the UAV when you open the music archive p0data62).
34: Zidane's Theme
70: Unrequited Love
9: Vivi's Theme
82: Black Mage Village
76: Fleeting Life
10: Steiner's Theme
62: Steiner's Delusion
29: Garnet's Theme
113: Melodies of Life
69: Over the Hill
77: Awaking Song // A Song from Her Memory, probably sung in Dali
115: Zidane and Dagger's Song
106: A Song from Her Memory
59: Distant Memory
124: Light of Destiny
32: Freya's Theme
68: Burmecia
50: Unforgettable Silhouette
15: Quina's Theme
88: Eiko's Theme
139: Girl of Madain Sari
2: Amarant's Theme
116: Roses of May
117: The Wavering Blade
125: Protecting My Devotion
30: The Evil Within
53: Court Jesters
133: Cid's Theme
118: Master of Time
84: Pandemonium
95: Kuja's Theme
96: Desert Palace // Kuja's Theme, played during the exploration of the Desert Palace
97: Devil's Ambition
98: Bahamut is Summoned
94: Wicked Melody
134: Dark Messenger
73: Moogle's Theme
4: Danger in the Forest
60: Ice Cavern
3: Village of Dali
67: Beyond the Twilight
74: Lindblum
49: Out of the Frying Pan
57: Qu's Marsh
108: South Gate
85: Dark City Treno
92: Cleyra's Trunk
80: Cleyra Settlement
91: Gargan Roo
79: Fossil Roo
90: Conde Petie
99: Before the Altar
130: Ruins of Madain Sari
81: Eidolon Wall
109: Iifa, the Ancient Tree of Life
112: Aboard the Hilda Garde
144: Esto Gaza
110: Mount Gulug
129: Daguerreo, the Hermit's Library
87: Bran Bal, the Soulless Village
132: Memoria
121: Crystal World
24: Aloha De Chocobo
100: Ukulele de Chocobo
22: Vamo Alla Flamenco
58: A Place to Call Home
36: Oeilvert
105: Chamber of a Thousand Faces
103: Ipsen's Castle
101: The Four Mirrors
131: Terra
44: Outlaws
138: Peaceful City // Outlaws, when controlling Vivi early disc 3
18: Swords of Fury
27: Find the Princess
20: Eye to Eye
17: The Fateful Hour
33: Thy Warmth
25: Tragic Love
143: Star-Crossed Lovers
41: Kiss of Betrayal
145: I Want to be Your Canary
142: Inseparable Hearts
65: Prima Vista Orchestra
75: Rufus' Welcoming Ceremony
12: The Meeting
140: The Black Waltz
83: Eternal Harvest
7: The Extraction
141: Foolproof Love Letter Scheme
86: Look Back, See the Frog!
146: Broken Spell, Healed Hearts
72: Guardians
48: You're Not Alone
147: Assault of the Silver Dragons
1: Unforgettable Sorrow
89: Mourning in the Sky
45: Another Nightmare
52: RUN!
111: Hunter's Chance
66: Tetra Master
0: Battle
35: Boss Battle
61: Faerie Battle
71: Final Battle
5: Victory Fanfare
6: Game Over
156: Prelude
148: Doga and Une
Hi Tirlititi and other members!
Beatrix Alexandria Battle was one of the fights that I wanted to change the music for.
I would also like to change six others:
- in the fight against the guardian of the earth, I'd like it to be the guardians' music that keeps playing.
- when fighting the Nova Dragon , I'd like it to be Assault of the Silver Dragons that keeps playing.
- the four guardians in memoria, once again I would like the Assault of the Silver Dragons to be launched instead of the classic boss theme.
I understood that each location had its own ID and each fight in a location had its own ID. So I have to find the ID of the place (temple of the earth for the first one, memoria for the second one) and then the ID of the fight in question (a priori there is only one for the temple of the earth but several for memoria) to replace the ID of the music (35 -> 72 for the guardian of the earth, and 35 -> 147 for the Nova Dragon and the 4 guardians).
But Idk how to find the IDs of the locations and the fights.
In fact I even wonder: for the 3rd fight against Beatrix, I managed to change the music following your instructions but first I wanted to search by myself in the "BtlEncountBgmMetaData.txt" looking where the "117" of Beatrix's theme appeared. I thought I could find the ID of the fight against Beatrix in Bluemecia and Clayria, and thus find the ID of Alexandria (if the "fight against Beatrix" IDs were the same from one area to another). And in fact, the ID "117" of her theme didn't appear anywhere. I think it's simply because in the first two encounters with Beatrix, the music was already playing before the fight started.
If my reasoning is right, if I replace the "35" with "72" and "147" in the fights that I'm interested in, the music will start playing again from the beginning when the fights start and it won't be great. So how do I fix that? For the 4 guardians in memoria, it's no problem to replace "35" by "147" because the music begin at the same time of the battle, but for the two others (earth temple and nova dragon) Idk how to prevent the music playing from the beginning (if my idea is correct about how "BtlEncountBgmMetaData.txt" works)...
Finally, one last question, for the 6 fights, I would like that the victory music does not start at the end of the fight and that the music of the fight continues until the end of the table of gains of experience, objects, etc.. Checking the "Don't Stop Music" box for each fight will allow this, right?
I hope I made myself clear ^^'?
Anyway, ty for your amazing works and ty in advance for your response !
If other people have the answer to my questions, thank you as well, of course!