This instructable is a guide on how to build an amazingly small, powerful phone speaker inside a coke can. Total cost of this speaker is below 20$, excluding batteries and coke can of course. Advantages of this speaker include:

Loudness
Stereo Sound (vs. Mono found on cheap phones, etc.)
Very Energy Efficient
Cool looking
Recycled
Very cheap compared to buying a speaker
You can add your own features
Long Battery Life

Step 1: Gathering Your Parts

For this project you will need the following parts:

2 x LM386 - This is our main audio amplifier IC. It is available basically everywhere and is very cheap. I paid around 40 cents for 10 of them, but that is china prices. In the US they go for around 1$ per ten of them.
4 x 200uF caps - These are the main filtering caps. You could go for expensive Japanese audio ones, but I found that the Chinese 'Chenbin' brand ones are good enough. Any cheap cap should do, as this is not some sort of crazy high end system.
2 x 100uF caps - These caps are used for setting the gain. Again 'Chenbin' brand it is.
2 x 1W 8R 50mm - speakers. They are super cheap, around 30 cents for 1. And they sound decent. For better audio quality, definitely get some better speakers.
10 Ohm Resistor - Just buy 100 of them, they are cheap and always useful to have.
2 x 10K potentiometer
1K Ohm Resistor - Again, just buy 100. You will use them in other projects.
1 x Audio Cable - To connect your phone
2 x 9V Battery Clip
1 x Breadboard - If you want to prototype your circuit.
1 x Coke Can

Step 2: Gathering Your Tools

This is a fairly advanced project, so you require at least a minimum selection of tools. These include:

A Dremel Rotary Tool. Preferably the Dremel 3000. We need this to cut the protoboard and the coke cans. Similar rotary tools from other brands might also work, but I can only recommend the Dremel 3000. (If I am referring to 'The Dremel', I mean the rotary tool.
A 3.2 mm drill bit for the dremel. (It should come with one)
Temperature controlled Soldering Iron. You can get a decent one for as little as 80$ if you don't have one already. I use the Hakko FX-888 and it is doing a great job.
A reinforced grinding piece for the dremel. (again, should be included in the set, otherwise the speedclick ones are great)
A hot glue gun. Mine is from Dremel, The Dremel 940, but any hot glue gun should do. Just don't buy the ultra cheap ones, as they can melt in your hand.
Wire snips.
Wire Strippers.
Craft Knife, Scissors
Needle Nose Pliers
Breathing Mask - To prevent you from breathing the dust while grinding.
Goggles - To protect you eyes from dust while grinding.

Step 3: The Basic Framework

Now we finally reach the building phase, after two rather long (and boring) parts and tools lists. So for step 3, it is to start cutting up the coke can.

Use the Dremel to cut the top and the bottom from the coke can. Only cut out the inside, so that the speakers fit perfectly. Also drill a hole to fit the aux input. Use a drill bit of the same size as the plug on your cable.

After this, move on to the next step.

Step 4: Assembling the Amplifier

Now we are getting to the interesting bit. Cut a 4cm x 4cm piece of Protoboard. This will be where your amplifier will be installed upon. Look at the pictures and schematic of the circuit. Use it to guide yourself in the installation process.

After finishing the two amps, use a small piece of protoboard and some wires to split the audio signal for the two speakers. Then build use protoboard, wires, battery clips and hot glue to construct the battery holder.

After this we can move on to the next fun step. Installing it in the coke can.

Step 5: Installation

Put a 9V battery inside the clip. Now start by putting the end of your aux cable through the hole in the can. (see pic. 1)

Put the rest of the installation inside. Starting with an amplifier, followed by the 9v battery, followed by the aux splitter and lastly the second amplifier. Ensure that the speaker wires that are attached to the amplifier are coming out of both sides of the can.

Solder on the speakers onto the cables. Be careful of the speakers polarity.

For the last touch, use some hot glue to fix both speakers in place.

Step 6: Enjoy

You have now build a amazing speaker. Enjoy it.

Some potential improvements for the project would probably some more expensive speakers, EMF protection and maybe a Bluetooth module. Also, a rechargeable battery might be nice.

If you enjoyed this instructable and think it is pretty darn cool, I would be happy if you could vote for this instructable in the seeedstudio rephone contest.