Doing so does not increase the file size proportionally since the black area compresses very nicely. The main benefit of audio is that it provides a channel that is separate from that of the display. Speech can be used to offer commentary or help without obscuring information on the screen. Audio can also be used to provide a sense of place or mood as done to perfection in the game Myst . Mood-setting audio should employ very quiet background sounds in order not to compete with the main information for the user's attention.
Music is probably the most obvious use of sound.
Whenever you need to inform the user about a certain work of music, it makes much more sense to simply play it than to show the notes or to try to describe it in words. These kinds of background sounds have to be very quiet and nonintrusive. Also, there always needs to be a user preference setting to turn them off. Good quality sound is known to enhance the user experience substantially so it is well worth investing in professional quality sound production. Many multimedia elements are big and take a long time to download with the horribly low bandwidth available to most users.