It depends on the application or context -
With an EEG or ECG, I'd say it's active and the diagnostic equipment amplifies and senses signals generated naturally by the body (if I understand how those work).
With an echocardiogram, the ultrasound signals are generated outside the body and the body is passive.
I suppose the body could be considered an amplifier (active).... If somebody says "lift this rock", there is not enough sound-energy to lift the rock but you can amplify that signal with your muscles.