Man this thing is finicky. I thought maybe it was outdated or just broken, but by using Audacity [www.audacityteam.org] after some trail and error I was able to export my mp3 into the right format (ogg vorbus or .ogg, 112 kb/s is the exact format StS uses internally, so that's what you're aiming for). Here's a screenshot of my export settings [imgur.com].
MOST IMPORTANT DETAILS:
- Ogg Vorbis, .ogg
- 44100 Hz sample rate
- Quality: 3 (this gave me 112 kb/s), but double check that that's right using VLC media player's [www.videolan.org] codec information by playing the file then hitting Ctrl + J or clicking Tools > Codec Information. Alternatively, MediaInfo [mediaarea.net] can do it but I don't remember the steps.

Also to add onto fleur (the mod maker)'s audio tracklist of file names, here's a fuller one that's sorted the same as the official soundtrack [clarkaboudmusic.bandcamp.com]:
STS_MenuTheme_NewMix_v1.ogg - Menu Theme (Slay the Spire)

STS_Level1_NewMix_v1.ogg - Exordium Theme 1 (Exordium)
STS_Level1-2_v2.ogg - Exordium Theme 2 (Battle Trance)
STS_Boss1_NewMix_v1.ogg - Act 1 Boss (The Guardian Emerges)
STS_BossVictoryStinger_1_v3_MUSIC.ogg - Act 1 Boss Stinger (Exordium Slain)

STS_Merchant_NewMix_v1.ogg - Shop Theme (Meet the Merchant)
STS_Level2_NewMix_v1.ogg - City Theme 1 (The City)
STS_Level2-2_v2.ogg - City Theme 2 (Escape Plan)
STS_Boss2_NewMix_v1.ogg - Act 2 Boss (Battle With the Champ)
STS_BossVictoryStinger_4_v3_MUSIC.ogg - Act 2 Boss Stinger (The City Slain)

STS_Shrine_NewMix_v1.ogg - Shrine Theme (The Unknown)
STS_ALTLevel_v1.ogg - Unused in game(?) (After Image)
STS_EliteBoss_NewMix_v1.ogg - Elite Theme [Used for Lagavulin] (Facing the Elite)
STS_BossVictoryStinger_3_v3_MUSIC.ogg - Elite Singer (Elite Slain)
STS_Level3_v2.ogg - Beyond Theme 1 (The Beyond)
STS_Level3-2_v2.ogg - Beyond Theme 2 (Dramatic Entrance)
STS_Boss3_NewMix_v1.ogg - Act 3 Boss (The Awakened One)
STS_BossVictoryStinger_2_v3_MUSIC.ogg - Act 3 Boss Stinger (The Beyond Slain)

STS_Credits_v5.ogg - Credits (The Credits)
STS_Boss1MindBloom_v1.ogg - Mind Bloom option. (Mind Bloom)
STS_Act4_BGM_v2.ogg - The Ending Theme (The Ending)
STS_Boss4_v6.ogg - Act 4 Boss (The Heart)
STS_EndingStinger_v1.ogg - Ending Stinger (The Spire Slain)

ALRIGHT! LISTEN UP EVERYONE!
I managed to make it work. This is what I did:

When you're converting the audio file to .ogg, make sure the conversion settings are:
-bitrate: 256 kbps
-audio frequency: 44100 Hz
-audio channels: Stereo
-audio codec: Vorbis

I am not sure if all of those are necessary, but converting this way made it work.

Not sure if the mod author is still active, but has anyone been able to get this mod to work properly? Seems that even following the instructions, I get the folder name to appear in the config settings in-game, but only get the base game music to play...

@Bio. @SpiralID
It seems that some OGG files can not played in Slay the Spire according to downloader and converter.

In the case:
- In VS Code editor, I can not open OGG file and play

- in Slay the spire and in-game log example:
=========================
06:09:26.189 INFO core.PrismriverCore> Found Custom Audio, Playing: STS_Level1_NewMix_v1.ogg from musicpack arknights_2
06:09:26.191 INFO core.PrismriverCore> Couldn't find Custom Audio, playing Basegame Default instead.
=========================

I have converted MP3 files to OGG using Python command (library): https://pypi.org/project/ftransc/
It works well.

Has anyone gotten this mod to work? I've been trying again and again, changing the names, folders and audio file type and all yet it doesnt work

Good mod! I found names of vanilla music in path audio/music of "SlayTheSpire folder - desktop-1.0.jar file".
so, I can change these!

I want to use the boss 2 music for all the other boss themes and that's it, will it still loop properly? also, where can I get the boss 2 music file the game uses so it's the same

OK i can't get this to work

Making a SongPack/MusicPack

Create a folder in the prismriver directory (StS installation/prismriver), this is your songpack folder.
Put music files into the folder, they must be .ogg format, and they must be named the same as the track they are replacing

Basegame Track names
You'll be using these to name your tracks you want to replace.

STS_MenuTheme_NewMix_v1.ogg - The menu theme.

Instructions unclear do i copy

STS_MenuTheme_NewMix_v1 in ogg format?
or copy STS_MenuTheme_NewMix_v1.ogg using the .ogg?
or copy all exactly as STS_MenuTheme_NewMix_v1.ogg - The menu theme.?

I've tried all 3 methods with no success.

Do i need specific encoding?
Do i need to use a specific converter?
Should i just rename an mp3?

I found enabled the folder fine, it just straight up refuses to work.
how do i find playing tracks?
I removed everything apart from this and required with no success.

@Praetorian Right Click StS in your library, go to local files, and press "browse local files". That is your StS directory.

where does one find the sts directory, i do not know what it is. help would be appreciated

Ah sweet, thanks for the info, great mod.

@Om3ga-Z3ro I'm trying to find a way to only show one pack through that selection to be selected. In the current case, only the last selected music pack is used.

For the random selection, please use wildcard mode. It picks through all music packs installed to find a song to replace. This is for every song that would play.
Please be aware - that if a wildcard song is generated and the song doesn't exist, it will immediately go to the default.
(It is under the intention that music-packs are a complete package and thus this problem shouldn't occur.)

Loving this mod
I have a question though, if you have multiple music packs and you tick all of them, will that make the music randomized or does it randomly pick one of the music packs and just play those throughout the playthrough?

@Snowdrift huh, I saw that music track in the files but never actually saw it anywhere. The file is STS_EliteBoss_NewMix_v1.ogg, and it's been added to the guide.

Which one of these is the Lagavulin theme? I think it might be missing from the list.

So I am trying to use this mod to replace music from the Gensokyo act pack mod. I noticed that some music files there are in the MP3 format instead of OGG. Would that mean I also have to match the format in the songpack folder for this mod as well?

@Horseman It's any music file, I've just mentioned the basegame because that's what I know, and I don't know the music-keys for modded tracks, however the mod writes logs which you can find at (SendToDevs->mts_process_launch.log). If you CTRL-F for the phrase "Generated Song" you should find the song's key underneath it.

Can you change any music file with this mod or just the mentioned ones?

@SolarNougat it is replaceable, I just forgot to put the ID in the guide. It is now added.

Thanks for making the mod

I noticed that there's no track name corresponding to the Act 4/The Ending boss. Is it irreplaceable by this mod?