Lets say you have a File called "XYZ.MP3" and you want to change/replace the Theme of Jangan Town.
Firstly you need to convert the MP3 File into a .OGG File. (There are several tools for that like Free MP3 wma Converter). Once you have converted the MP3 File to the .OGG File you need the Theme Names, in that case, Jangan Town theme is called "jangan_town".
Name the .ogg File "jangan_town". Open Joymax Tools or another PK2Editor and select the Music.PK2. Replace the Theme and it is done. Also you shouldn't import Files over 15 MB otherwise your Client might crash when it tries to play the Theme.