How to overide stuff from the core (like the entry song)

[Vantave]

Hi, I'm making a music mod and I have one question:
How do I override core game stuff? Like if I want to change the menu song, how would I stop the other one from playing without getting a ton of "Missing Song_Entry def" error messages on launch?
Also how would I stop the base game music from playing and instead replace with my own music?
Thanks, also I'm completely new to modding and only know the very basics of RM modding, so keep it simple