It's pretty simple, you just need a few electronic parts, some wires, LEDs, resistors, diodes and a switch.

Can be made for under 20 USD

Here's what you need:
LED (Any color)

Battery 3.8V

Amplifier (PAM8610 Digital Amplifier Board, is a good choice)

Diode (Any above 5 volts)

Resistor (180 Ohm)

USB Bluetooth Music 3.5mm Audio Stereo Receiver for Car AUX IN Home Speaker (After what I've found online that one should be the best)

5V Micro USB 1A 18650 Lithium Battery Charging Board Charger Module New BU

3V to 5V 1A DC-DC Converter Step Up Boost Module USB Charger

2 Loudspeakers

Switch (Any kind will do)

5 Meters of copper cable

Step 1: Solding Battery Charger Onto Battery

Required for this step:

3.8 Battery

Battery charging module

2x 7 cm copper wiring

This is VERY dangerous, do not ever sold directly on the battery, take your copper wire and hold it onto the battery "+" drop the soldering tin 5 mm above it and let the soldering tin drop onto the copper cable so it will stick to the battery, now sold the other end of the copper cable onto the "B+" on the Charger. Do the same for "-" and "B-"

Step 2: Making the Switch

For this step you will need:

Last step

Switch

Copper wiring (2x 5 cm)

Wire a 5 cm copper wire onto the Out+ on the charging module, now wire the other end on the + on the switch (Usually the middle of a switch)

Wire a 5 cm copper wire onto the Out- on the charging module, now wire the other end on the - on the switch

Step 3: Making the LED Indicate on & Off

You will need:
Last step

Diode

Resistor (180 Ohm)

Copper cable (5 cm)

Wire the 180 Ohm resistor on the - of the switch, now wire the LED - on the other end of the resistor, now wire a diode on the Out- on the charging module (with the direction indicator facing towards the charging module) now wire a copper cable underneath the direction indicator of the diode, now wire the other copper cable end onto the + leg of the LED.

Step 4: Stepup Installing

For this step you will need:

Last step

Stepup module

Copper wire (1x 10 cm & 1x 4 cm)

Wire the 10 cm copper wire on the IN- on the stepup, now wire that to the end of the diode from before (The part that is not facing towards the charging module) Now take the 4 cm copper wire and wire that onto the IN+ on the stepup and wire that onto the - section of the switch (Before the resistor, but on the same line)

Step 5: Bluetooth Installing

For this step you will need:

Last step

Bluetooth module

Stick the bluetooth USB into the stepup USB port

Step 6: Amplifier Installing

You will need:
Last step

Amplifier module

Copper wire (3x 7 cm & 2x 12 cm)

Connect the - on the amplifier to the - on the stepup with the 12 cm wire (Notice the - on the stepup is on the bottom of the module) Do the same with +. Now connect the Right mini jack section on the amplifier to the Right mini jack section on the bluetooth module using a 7 cm copper cable, do the same with Ground and Left.

Step 7: Speaker Install

For this you will need:
Last step

2x loudspeakers

4x 5 cm copper cable

Take your 2 loudspeakers, pick any of them to be left and right, now take the left speaker, and connect the - on the speaker and connect it to the Left - on the amplifier, connect the + on the speaker to the Left + on the amplifier, do this with Right too.