The Sims 3 has no built-in "Add Music" button. The game reads proprietary .sng/.sn3 packages from GameData/Shared/NonPackaged/Worlds/.../Music. However, three practical workarounds exist:
Documents/Electronic Arts/The Sims 3/Custom Music (create folder if missing).Pop_01.mp3, Classical_03.mp3). List at Sims Wiki.Limitation: Only works for the "Custom Music" radio station. Other stations (Pop, Rock, etc.) still play EA tracks unless you mod FullBuild.package (advanced).
Install via Mods/Packages (enable "Custom Content" in launcher). Always backup Saves and DCCache first.
Only use music you own or have licensed. Streaming-service audio is for personal listening—don't redistribute modified game packages containing copyrighted tracks.
Recommendation: Start with Method 1 (Custom Music folder). If you want per-room control, add NRaas. For zero hassle, Method 3.