You need to create a mod.

The simple way is to create a new folder in your mod folder, then create a "Sound" folder. 

Then go to the game install folder and open the Sound folder there. Listen to the files that start with Music. Find one of your tracks you want to replace it with, and paste your track into your mod's Sound folder. Rename the copy of your music to the same name as the one in the install folder.

Enable your mod and you will have replaced the default music with your own.

If you want to add totally new tracks along with the existing ones, that is more difficult as Sins does not just play music randomly, but tries to calculate the mood and intensity of the game, then selects an appropriate track. It requires editing several files in addition to placing the music in the sound folder.