Music Mixers (commonly referred to as keyboards) are an object in The Sims 2 which can be used by your sim to create music. They can only be found in the Lion Lounge and Sax Lounge, and can't be purchased from the Store.
When using the Music Mixer to make a song, you can choose to use some or all from a seclection of the pre made game songs. As well as this, you can also record your own short sound via the microphone which can be used for the sound on the keys of the keyboard. Whilst in the process of making music if there are sims in the lounges, you may receive a tip of around 15 simoleons. A maximum of three saved songs can be stored per game, although they can be replaced if you make a new custom song.