Sound investments refer to investments in audio equipment such as stereos, amplifiers, and hi-fis. Stereos are seven letter words that refer to a device that plays sound from a variety of sources such as CDs, cassettes, and radio. Amplifiers, or amps, are four letter words that refer to devices that increase the power of an audio signal. Hi-fis, or high fidelity systems, are five letter words that refer to audio systems that produce high quality sound.