Here's a new article with an imagineer talking about SM's audio:
http://thedailydisney.com/blog/2010/09/space-mountain-music-hear-all-about-it-from-an-imagineer/
Like many of you, I've always felt that the lack of on-board audio was a simple budgetary decision.
But this Imagineer makes a good point about the unique rockets on SM. I can only think of a few coasters with the same low-profile, single-file seating.
I agree with him that a headrest would alter the WDW SM experience (And no, I don't buy the theory that speakers could be mounted on the trains without a large headrest. I am 6' tall. On DL's SM, which has a stubby headrest, by head is still so far above the speakers that my ears mostly pick up the track noises, not the music)
I think Martin said they tested an on-board system on the existing WDW track. But an Imagineer-type recently posted on another message board that it is difficult to implement on-board audio on a pre-existing coaster.
I assume it would have been difficult to add on-board audio without replacing the entire WDW track. Some might call it cheap that they didn't replace the track. I call it wise. If the track was safe and in good condition, there is no reason for Disney to spend big $$$ tearing it out and replacing it just to add music. (As I've pointed out in previous debates on this subject, you would not tear your house down and rebuild it from the ground up simply because one interior wall was not sturdy enough to hold some speakers for a new home entertainment system.) And remember, WDW would have to replace not just one, but TWO tracks.
I also agree with this Imagineer that the dome audio in WDW's SM adds to that attraction's uniqueness. You really cannot ride a coaster that way anywhere else. If they had gone with on-board audio (especially with a re-built track and new trains) I think SM would have been way too similar to Rock-n-Roller Coaster.
The new WDW SM audio system may not be as dramatic or as high-tech as DL's onboard system, but it makes a fun ride even more enjoyable, while also giving visitors a truly one-of-a-kind experience.