This boombox based on cheap parts and had a 3D printed body.
I've made audio level and battery indicators by myself but you can use complete cheap modules like this:
2S Battery indicator
Audio level module
Passive speakers works well only with a leakproof body. To prevent leakage you must glue speakers and all halls in the acoustic volumes with a silicone mastic or some resin glue.
18650 batteries connected together in a 2s2p schematic and work with th BMS protection board which can control high and low charge levels and have short current protection.
You can use some special boards for charging 2s battery or connect battery (with BMS) through a serial ballast resistor 5 W ~5 Ohm directly to 12 V or to Step UP voltage regulator (USB 5V to 12V), I use this way and it works fine but produce a lot heat.