Unfortunately that is the case, there is not much that can be done about it. Since the male voice is usually deeper and deeper tones contain more bass, which creates stronger air movements, this can also be better picked up by microphones. This is generally the case and is based on the physical explanation of the propagation of noise. If the microphone were to be set to be more sensitive, ambient noises would also be perceived more intensely and would be more difficult to filter from the actual speech. In addition, the microphone could easily be overdriven in noisy environments, so that Pepper would no longer understand anything.
(the sensitivity of the Migrophone cannot be regulated by yourself in order to avoid these situations and to equip Pepper for all areas of application equally (whether loud or quiet))
Therefore, one can only ask people with a higher voice to speak a little louder.