Don't take this the wrong way--I'm disappointed there will not be on-board audio.
But one drawback of audio is that fans like us are likely to memorize the music. You can hear it playing along in your head as you're riding. And that means on a pitch-black ride in the dark like SM, you know where you are on the ride path at all times, you can anticipate the drops and other synchronized motions, and you know how close you are to the end of the ride.
On most coasters, like BTMRR, you have a visual sense of what's coming up. I know that after going up the avalanche hill, the train will begin slowing down, and my ride will be ending very shortly.
One thing I love about SM (and maybe I'm in the minority) is that no matter how many times I've ridden, I can't seem to "memorize" the track, and elements like drops or the re-entry tunnel always surprise me. The ride always seems to go longer than I remembered.
When I rode DL's SM for the first time, I was already familiar with the music. So I knew what to expect. (Of course, that's true of any attraction soundtrack you memorize. But for some reason, SM is unique because the audio is you're only cue).