Speakers on the ride vehicles have become very unreliable on California Screamin' and Space Mountain at the Disneyland Resort. They just don't work regularly (although the big problem was pin trading/ haunted mansion letterman jacket yeilding AP holders who seem to complain about anything, and insist on being seated in a certain train because the sound works on that one only, which is why Disney turned off all speakers). Both Speaker systems have been added lately, and are not reliable. They do enhance ride experience, especially on CA screamin IMO. CP buying speakers for 6 trains would be expensive, and would return little money. Having video cameras mounted on the seats (which has been tried by disney on Superstar limo, but not video, just stills) would not be all that much more expensive and would be more likely to return their invested money.
As for Vibrations, Xcelerator's roughness is the vibrations.