Sound tests are a traditional extra in video games. Offering a quick way to listen to any of the sound effects or background themes in the game, these are usually leftover development code or unlockable extras (the latter being more of a recent trend). While most gamers overlook them entirely, with good reason, certain games have music of quality worth listening to independantly, and this is a good way to hear it. Additionally, you can just keep doing a sound effect over and over to annoy someone. Whatever suits you.
A sound test screen usually looks like this:
> Sound: 01
Generally, not a lot of effort is put into these screens, as few gamers are interested anyway. Some games lack the option to play sound effects, and a rare few lack the ability to play music. Occasionally, a screen will be barebones to the point of even having the songs/sound effects organized by hex codes, but this is less common.
As a rule, sound text modes are hidden on the "Options", "Controls", or "Bonuses" screens in your game. A surprising number of games have sound test modes hidden behind cheat codes; a quick review of a site like GameFAQs will turn up any codes like this, if they exist.