The MoHa E231 is a motor car with pantograph for the JR E231 train set. Despite being designated a motor unit, it does not make the sound in the E231 train set; however for the moment only the 'locomotive' of a train makes noise in advtrains, so this is how the sounds have to be handled.

For realism, you should always include a KuHa E231 in your E231 trains, because it has the pantograph for power pickup. Otherwise you would have to ask “how is the train getting power?”.