Instead of a microphone storage compartment, which is not there, it would be better to make a compartment for a single 18650 battery. Batteries 4 pcs were enough for us for 20 days. I soldered an 18650 battery (from an old laptop) through a special scarf, now one charge should be enough for a month, according to calculations. At the moment of pressing the key, the peak current consumption is about 100 - 150 mA, while playing a melody up to 200-250 mA. It is strange why the manufacturer saves on this. Batteries are not serious.