so first its important to understand WHERE the music is actually located.

i'm sure most people who read steam guides already know but incase you don't, to get into the files right click risk of rain returns in the library, hover over "Manage" and then click "Browse Local Files". this will open file explorer. i don't know why, but all the music isn't really buried in a labyrinth of files, its at the VERY entrance, so no need to go digging further.

the musicBoss files are for boss fights, with a number and a, b, or c. musicBoss3 is dedicated to the soundtrack playing in providence's fight, with musicBoss3a being phases 1-3, musicBoss3b being phase 4, and musicBoss3c being that one song that plays for most of the trial of judgement and stage 5's teleporter event, for some reason. all of musicBoss1 and musicBoss2 are just teleporter event music.

musicCredits is the credits, i don't think i need to explain this one. musicHitItSon is from that easter egg in Risk of Rain (the level). musicStage 1-10 are just stage soundtrack, but for some reason musicStage5 and musicStage7 are missing for me. i'm guessing they just reused music for two stages.

musicTitle is the title screen, with musicTitle2 being the original RoR1 title that plays in the character select. wRainAmbience is just rain. i'm guessing that it's used in Risk of Rain (the level) since that, obviously, has rain.

now, lets move on to how to REPLACE the music, atleast my methods of doing so. if you already have a method, great! just make sure its a .ogg and i've told you all you need to know. otherwise, lets move on.

STEP ONE
first we need to get our hands on the music in the first place. any audio type will work since we'll be converting it anyways, though .OGG files will save you some time since you won't need to convert it.

if you already have a method without any flaws to get an audio file, go ahead and use it.
there are two methods i've used, though i think this one is vastly more efficient. cobalt.
first, your gonna need a youtube link to the song you want. got it? good.

next, your gonna wanna go to cobalt.tools and go to "save" on the top left. i'm pretty sure your already in the save section by default when loading into the site, but just in case, that'd be where you find it.

paste the youtube link into the bar that literally tells you to paste links into it. select "audio" under the bar. once you paste the link into the bar, you should see an arrow on the right side of the bar. click it, this will start a download of the file on your browser.

now that you have the file with the song you need, we can move on to converting it to a .OGG file.

ALTERNATE STEP ONE
just record it. this method is better in how it can disregard outros and intros without cutting those out with an editing software, however its a lot more inconvenient and human error will make you want editing software anyways. this is basically worse in every way.

STEP TWO
next, we need to convert this file into a .OGG file, with the song attached. i recommend VLC media player. i am too lazy to tell you where to download this, there's plenty of guides on youtube already.

open up VLC media player. in the top left corner there will be something that says "Media". click it, then click "Convert / Save"

next, your going to select any file (to my knowledge) with audio. this means it works with videos, audio files, ect. then click Convert / Save again (in the Convert / Save menu, yes, i know)
this opens up a new file with a bunch of stuff we don't care about and the "profile", which is basically a bar which you can select file type with. now select "Audio - Vorbis (OGG)". then select destination file and start.

this is where the jank (atleast for me) begins. NAME THE NEW FILE SOMETHING ORIGINAL IN THE DESTINATION SELECTION. the primary cause of jank is the file "already existing", and i'm just letting you know, your gonna want task manager for this. if you NAMED THE NEW FILE SOMETHING ORIGINAL, it will show its progress in conversion. once the process ends, it will pull up endless errors saying it exists already. it exists, however, because it just made it. click "End task" twice quickly in task manager for VLC media player, and if all went well you should have a .OGG!

now if all went well, you have a .OGG file with your music in it. go into the risk of rain returns files (how to do so in first section) in one file explorer tab, and the area you put your .OGG file in with your other. now, take note of the EXACT file name, case sensitive, that your replacing. now delete the file, and drag your .OGG file in. if your not sure that its going to work, create a backup of the file your deleting. name your .OGG file (in the Risk of Rain Returns files) the EXACT name of the previous song you just deleted.

for example, if i had a file called ReallyCool.OGG, and i wanted to replace musicBoss1a, you would take note of the name of the file your deleting, in this case musicBoss1a, delete it, drag ReallyCool.OGG into the Risk of Rain Returns files and name it musicBoss1a (the name of the old now deleted file). if all went well, ReallyCool.OGG should now play.

Good job.