I don't even know if this would require an Arduino, but the idea is useful enough. It probably might.
What I have is a Christmas item that has two ON switch positions. One position has lights and plays music, and one just turns on the lights.
What I want to do is mount a micro SD card reader in it, something like this:
I'd like to hook this up to the power and a speaker so that when the switch is flipped to just lights, it will play whatever mp3 file is in the SD slot.
So you can turn it on and hear the pre-installed music, or turn it on to the other position, flip on the SD slot, and it will automatically play whatever music you have on the card.
Does that make sense?