Here are the latest news.
I worked on a lipo battery system, charging option and a audio amplifier.
The battery system works great. I use a 7,4 V 2 cell lipo battery together with a simple balancer charger circuit.
I installed a voltage regulator for 5V and soldered everything together on a stripboard.
The Audio amplifier is a standard circuit with a tda1519 ic.
I installed two small speakers inside the cube. So everything fits inside.