First, get your music and make sure its in 44100 sample rate and it's a .mp3, you can use free software like audacity to load it in to check the sample rate as well as export it into a new one and/or convert it into a mp3.

Make a folder in garrysmod/garrysmod/addons and name it whatever you want, we'll call it TA_MyMusic for this example. Make an entire folder file path for these 2-3 paths in that addon folder. 3rd is optional and is only there for coverart.

TA_MyMusic\gamemodes\TemporalAssassin\content\sound\TemporalAssassin\music
TA_MyMusic\scripts\vehicles\TemporalAssassin\music
TA_MyMusic\materials\TemporalAssassin\music (optional)

In the Music folder for each of these file paths you're gonna make a new folder named whatever you want, try to keep it unique to your music though. Make sure ALL NEW FOLDERS HAVE THE SAME NAME. For this example we'll name it "MusicJam".

For example :
TA_MyMusic\gamemodes\TemporalAssassin\content\sound\TemporalAssassin\music\MusicJam

In the MusicJam folder for the Gamemodes file path (the first one) you want to put all your mp3s there. You then want to name and number them accordingly and define them. So if you have 3 tracks that you want to generally have while playing and fighting you could name them "Example01" "Example02" "Example03". If you wish to have tracks for when you're paused, at the hub or with dynamic music, gameover, high-difficulty gameover and credits then simply add one of these to the end of the file's name.

_PAUSED
_HOME (counts for calm tracks via dynamic music)
_GAMEOVER
_GAMEOVERHARD
_CHAPTEREND (credits)

So those example 3 files would instead, if we wanted 1 of them to be for home / calm dynamic

Example01.mp3
Example02.mp3
Example_HOME01.mp3


Now that you have your actual music files setup head to MusicJam in the scripts filepath (2nd path) and make a .txt document with the same name as the folder, in this case MusicJam.txt and inside that .txt you want to name the pack, this .txt dictates how the music pack's name shows up in the list when choosing between different packs.

After that you need to make a entire set of .txts named exactly the same as every music file you have in the first folder you setup. So if you have Example01.mp3 you need to have a Example01.txt in this folder and this goes the same for the unique tracks, Example_HOME01.mp3 would have a Example_HOME01.txt. Same case as the MusicJam.txt, you name the music files here. So if Example01.mp3 is, for example, Linkin Park - Numb. You'd likely open Example01.txt and type in Linkin Park - Numb, it will then show up in the hud when you select the Music pack.

Now for the 3rd and optional filepath, this is exclusively for "cover art" that being a picture that shows up when you swap music or hold scoreboard. Go to your MusicJam in the materials path (3rd path) and get a .PNG format image, set it to 256x 256y resolution and drop it into that folder, rename it to the same name as the folder and that's it.


ADDITIONAL NOTE : If you want to swap off Workshop TA to Legacy TA, aka manual folder installation of Temporal Assassin, you need to copy the .txts and the .png and put them ALL in the same folder with the music files.