Sound reflects from 'surfaces' (including density changes inside a body, such asthe difference between heart wall and blood). The challenge is that if there is a very strong reflection -- such as between air and a hard, dense, door -- it will be hard to tell much detail behind it. Imagineshining a light through a very heavily tinted window and trying to make out the details of a stained glass window behind it.
if you can place your emitter/sensor tight against a hard (and solid) surface, maybe you could pick up large objects near it on the other side? I'm not an expert, but as a GM I might give a roll at -4.
Sound carries better in water, and bodies are similar in density to water, so what things like dolphins can see is quite different than what you'll get in air.
The other to keep in mind is that sound is slow enough that the frequency of an echo will be changed if something is moving toward or away from you, so with the right analysis it is quite good at that sort of motion detection. I seem to recall that bats make use of that, and that is what basic ultrasound motion detectors look for.