I'm remodeling my basement and have a cold air return for the main level that hangs too low. I was wondering if I can run the return under the concrete floor, and if I can what should I use for materials?

nope to much work.
The issue with return ducts under a slab is that they must be made of either plastic or cement pipe. YOu simply cannot get pipe that large at least in a practical world. The size of the duct will be the first issue. It must be very large to allow the air flow to get back to the furnace. It also must be completely water tight. As the size of the pipe will bring it below the levels of water proofing thus the water table may become a factor. Als vapor tight prevents any smells, or radon from entering into air stream.
I would suggest that the return duct be moved and fabricated to run along the side wall either as a soffit or designed with diffrent measurements but keeping the same cross sectional size. The framing can be blocked off called panning to make the cavities act as cold air returns. Just be sure that no wires are run inside of these chases.
Check with a contractor that does duct work. I am sure he or she can figure out how to make this work.