I would like to find a material which could very noticeably reflect or absorb a sound wave frequency between 20Hz and 22kHz, preferably in a grassy or organic nature environment. Preferably this would be a rubbery or plastic material, if not a circuit component. if you know of a circuit component that would also be great.
The best reflective return from an acoustic source in air will be generated by something very dense, so as to yield the biggest acoustic impedance mismatch between it and that of air- ideally like a ball of steel. If plastic, use a hard and dense grade, like that used to make bowling balls.
For a circuit component, the best thing would probably be an extra-thick glass/epoxy or fiber/phenolic PC board- the thicker, the better.