How is it more trivial with sound? Sound is just a wave just like ultrasound. In fact, ultrasound has the word sound in it making it sound. So your conclusion is not sound.
Ugh, yes. Not sure how the wires got crossed like that. However, sound is just a wave while RF is also waves even if at higher frequencies. Just as ultrasound is higher frequency that audible sound (which I guess is how "sound" was being used). If you continue increasing the frequencies past the RF range you will eventually get into light.