I searched this for maybe a couple hours on multiple search engines and came up with nothing. It only told me how to ADD music, not REPLACE it. I already know how to add music and have known how to do it for years, and that is ABSOLUTELY not what I want to do.

What I did was I added my own MP3s to the build/buy and CAS music folder.Then I went into the game and went to the audio options and unselected all the sims music I didn't want playing. Then I restarted the game and now only my music plays when I'm playing the sims.

I'm not really sure if this is what you mean by replacing the sims music. I'm not even really sure if my little description above makes any sense. Hopefully, someone with more knowledge will be able to help you.

That's exactly what I did. I don't know if there is a safe way to delete the sims music, but unselecting it gives you pretty much the same outcome.