Mmmm ... even totally blind smart people would notice that the Sun's infrared radiation travels faster than sound.
They could probably build instruments and arrive to a world view not very different from ours, much like we have to build instruments to "see" gamma rays, X-rays or even radio waves.
And we do have an organ that detects gravitational force (or rather, its acceleration effect on masses): it is the semicircular canal, the little accelerometer in your inner ear.
Also relevant: Occam's Razor