This project could work great with a second hand pair of computer speakers.
You will need:
Munny Dolls. (x2).
Hot Glue gun
Vacuum Former (ok, so you prob dont have one but you will need a case for the amp)
Step 1: Breaking things
The Amp circuit board should also be taken out if you are going to make another case to house it. If you can/want to use the existing box, then leave it all connected. In hindsight, I probably should have done that rather than go to all the effort of making a new case.
Step 3: Template for speaker
Step 5: Cut hole for the speaker.
Repeat for both dolls.
Test for fit and trim as needed.
Step 6: Install speakers.
Bring the insulated wire up through this hole, through the neck and out the 'face'.
Solder wires onto the speaker.
Stuff cotton wool into the head cavity for sound dampening.
CAREFULLY glue the speakers in place and leave to dry.
Step 7: Make base
There are Instructables on how to make your own vacuum former and I reckon they are a great addition to any workshop.
Larger Munny dolls could maybe house a battery pack and the amp circuit. Depends on the circuit.