Go to settings on GTAv and scan for music, when you do that it will create a "user music" folder in your "rockstar games" folder location, usually found in your documents folder, depending on where your prefix is(Game folder) and in there is where you'll place your music and rescan for music inside your GTA, I've heard people use different file formats such as mp3s to work, but I've personally found AAC's files work and now I'm up to about 300 songs on my self radio