Alrighty, Im home so I have the files in front of me now!
As rjhughes stated, you need all the music you want to add in .ogg format. If you dont have a converter Google for some. Once you have them converted make a new folder and lable it Sounds. Also you will have rename them all. I get to that in a sec though.(We are going to make it like rj also suggested a mod, just incase we screw up something. 
).
Next step is to make a new folder and title it Sound Mod or something just so you know what it is. Stick you Sound folder from eariler into it and also create a new folder called GameInfo.
Go open find your SINS folder and look for the GameInfo folder, inside that folder you looking for 2 files:
Game.musicdata
SoundMusic.Sounddata
Copy both these files into your new Mod's GameInfo folder. Open those copies up with notepad or something like that. Lets start with the SoundMusic file.
When you open it you will see something like this:
TXT
numMusic 74
music
name "MUSIC_FRONTEND"
fileName "Opening Theme.ogg"
isLooping FALSE
actionLevel 0.5
actionRange 0.5
emotionLevel 0.0
emotionRange 1.0
To add music you will need to edit the file by adding your songs to this list. Here is where you need to name your files. If you noticed that the game already has for example, 14 battle songs...You will need to name any song you want to play during a battle this:
Battle_XX
With XX being any number 15 or higher.
Repeat this for the different songs you wanna play. Note: if you want the song to play during a quite time or sad time just drop the 'Battle' for whatever style your add it too. Look at the file and copy what you see, but you gotta use the types given.
So, you got your song all named now it time to add it to the list! Here what you should add if we use Battle_15 as an example:
music
name "MUSIC_BATTLE_15"
fileName "Battle 15.ogg"
isLooping FALSE
actionLevel 0.6
actionRange 0.4
emotionLevel 0.0
emotionRange 1.0
Note: all the action and emotion lvls and ranges can be just copied from other songs already in the file. They have meaning on when and how your song is played but thats a whole another discussion.
Anyhow if you go back to that first example I posted and the top of that list you will see this:
TXT
numMusic 74
This is very very importaint!!!! Each time you add a song to the list you need to add 1 to this number. Get this number wrong you get either a Minidump or that song is just skiped. Not good either way.
Go ahead and finish up adding the songs as you see fit. Make sure it look just like you started with minus the addtions of course. Oh and double check that number!!! Once that is done save the file.
Next open up that copy of Game.musicdata
Here you will see another list of music. Edit this file to include your addtions in the last one. For example again we will use Battle_15. So after this line:
musicTheme "MUSIC_BATTLE_14"
You add this:
musicTheme "MUSIC_BATTLE_15"
Again fill in your song addtions and save.
Last but not least once you have all that edit and your song converted and named. Go throw your Sound Mod folder into the Mod folder for whatever version your running. Dont know where that is? Start your game up, go to options, hit the Mod tab near the bottom and click on show path.
Once you got that settled, enable your mod and give her a spin!!!! Now you may not hear your songs for a bit since you add them to the end of those file list and even then it seems to be random at times.
Hope it works and you enjoy!
