I think that the reason babies move around a lot when we are being scanned it because...babies move around a lot! At my 12 week my baby was having some quiet time and the sonographer couldn't get the measurements she needed so got me to wiggle my hips (as if I was doing the twist while lying down) and the baby woke up and started moving around loads. So surely, if it was the sound or heat it makes/generates then the baby would have been really active without me waking it up! I don't think it would really be able to hear it logically though, a babys hearing would never be anything like a dogs (who probably could hear it!) Some people have loads of scans, and usually they are the ones who the medics are worried about. I wouldn't worry!