Why would you go through this much trouble voiding your warranty when 24v DC to 120v inverters exist: https://www.amazon.com/24v-120v-inverter/s?k=24v+to+120v+inverter

Also how do you know you can just send 24v to it? I assumed most of these size speakers were 3S wired cells (as some Partyboxes can run off 12V DC directly), so around 11.1V internally and possibly a 14-16v power supply for charging. You're taking quite a risk of breaking it or literally blowing up the LiIon cells by doing this.

Also on this note, the Partybox 310/200/300 can take 12v DC input directly to power it, maybe you should sell yours and buy one of those? https://www.amazon.com/Car-JBL-200-300-Power/dp/B08FD417DL

Electronics technicians would likely have to measure each pin and follow circuit board traces to figure out what each of those pins do. Nobody outside JBL will be able to answer that question for you.