The sentence refers to the inability of hearing as it asks to speak up or speak loudly.
Thus, "can't" in option C is correct. The modal verb 'can' indicates the ability or possibility of an action. The contraction 'can't' indicates 'cannot' which implies inability or impossibility. Thus, this matches the context.
"Can" in option A indicates ability, whereas the context implies inability. Thus, this is incorrect.
"Could" is the past participle of the modal 'can'. This is incorrect.
"Couldn't" in option D is incorrect as 'couldn't" is the past participle form of the negative modal 'cannot'. The context refers to the present tense, so past participle cannot be used.
Thus, option C is the correct answer.