When teaching English to Japanese students, it is important to remember that one of the key points to learning is volume.
Use your loudest voice possible, as it aids in the learning process. If the student asks you to be quieter, it is an indication that they are hesitant about learning English. If possible, speak even louder. If you cannot, move closer to their ears.
It is your duty as a teacher to aid the learning process in every way that you can.