Add Custom Music to Your Sims Game

Are you sick of the default music in The Sims 2 and 3? Add your own favorite music to have a more personalized experience!

Steps

Choose the music you want to include. It must be on your computer and encoded in either the .wav or .mp3 format.
Find the Sims 2 music folders, located at "C:\Documents and Settings\<yourusername>\Documents\EA Games\The Sims 2\music".
Each radio station from the game will have a folder. Copy the music you like into whichever folder you choose. Split it between folders to create playlists, but do not create new folders or delete anything.
Launch the game and go to Audio settings.
Find the radio station playlist where you put your music and uncheck any songs you don't want to listen to, such as the default ones.

Choose the music you want to include. It must be in the .mp3 format.
Add it to the Custom Music folder. This is located at "C:\Documents and Settings\<yourusername>\Documents\Electronic Arts\The Sims 3\Custom Music". You may delete the default music included in this folder without causing a malfunction.
To ensure the default music is deleted, go to C:\Program Files (x86)\Electronic Arts\The Sims 3\GameData\Shared\NonPackaged\CustomMusic and make sure the folder is empty of all music, including yours.
Open the game, and go to the Music Options menu by pressing the "..." button. Within that menu, click "Options" and then the musical note symbol at the top. Go through your playlist, making sure it is full of the songs you want to hear.

For The Sims 2, this method also works for the build mode, buy mode, and neighborhood music.

Do not rename folders as the game is programmed to open certain folders. If it can't find them, the game will crash.
Deleting files and folders that were installed with the game may cause the game to crash, requiring you re-install it. Do not delete any files in The Sims 2 and only delete custom music files in The Sims 3.
These methods only work for the PC versions of The Sims 2 and The Sims 3.
Neither Sims 2 or 3 will recognize .m4a files. Only use MP3s.
DO NOT add files too big, this will make the game unplayable, even AFTER you remove it again. Avoid using music files which lasts for more than 10 minutes.