Bird appears to be in good feather quality and health and also looks like a young drake to me. That explains the voice. I would not give antibiotics without a veterinarian's recommendation. Looks like a normal healthy bird to me.
:welcome Are you sure that the muffled quacking bird is not a drake ? Can you post a picture. If young enough it may have not molted into its nuptial plumage. Drakes generally have an olive colored bill and hens a brown/black bill.