Most of the included speakers Apple supplies in their hardware are “decent” quality, especially in the iMac line. In most cases, the internal speaker is of a quality good enough for beeps, alerts, and voice, but beyond that, music playback is only so-so because audio quality, tone, and loudness are all very subjective aspects of a sound system. The recent iMac line has speakers (stereo), that are good enough for watching DVDs in a small room; we use it for that purpose, and it works great.
Because Apple plays host to many multimedia applications (their forte IMO), most professionals that deal in multimedia go for professional spec speakers, and they do so with varying degrees of quality and preference (and price).
Visual displays, on the OTOH are integral to the experience of the OS for conveying resolution, accurate color, refresh rate, etc. so it is logical to assume that Apple wants that element to be spot-on in terms of consistency, so that is where they focus most of their attention.