Have you thought of using 5v usb power for the lights and player? You can get a few chargers and recharge them in a locker.
I have totally gone from 12 v 'car adaptor' outdoor camping with 12 v battery, electrical to 5 V LED with those flat chargers that last for several hours, and is really tiny.
They might get weird about the batteries even being sealed. It is just a thought and you have plenty of time to research it!
There are some badass little USB bluetooth speakers that last for many hours, are quite loud, some have LED lights and also can be recharged with those 'phone charger' devices. Just bring a USB hub for multiple outlet charging out of the locker. Bring a Bluetooth enabled music playing source, or a simple MP3 player with a headphone out and plug directly into the speakers.
5V is the new way. Just a thought.