This is a neat project I picked up about a month ago. It's a simple project allowing you to transfer sound across a space on light with little quality loss. The credit of this project goes here

Step 1: Gather Your Materials

Things You Will Need:

Two Mono Jacks
1 Audio Transformer
1 Solar Resistor
1 Laser
1 Single AA Battery Clip (for reciever)
1 Triple AAA Battery Clip (for laser)
Batteries (1 AA, 3 AAA's)
Some wires and tape

A breadboard is optional, but I chose to use one to save time.

Step 2: Add a Mono Jack

Start by adding two wires to the leads of a mono jack. This will be the input of your transmitter.

Step 3: Add the Transformer

Next we add the first two wires of our audio transformer. Connect the red and white leads of the transformer to the mono jack.

Step 4: Connect the Other Leads

The blue and green leads need to be connected to the breadboard and will later be connected to the laser. The middle black lead will not lead to anything, so it's best to wrap a piece of electrical tape around it, as I have done.

Step 5: Complete the Transmitter

Next we add the laser. The green lead of the transformer connects to the negative lead of the laser, and the positive lead of the laser leads to the positive lead of the battery. If everything is connected properly, you should be able to turn your laser on. This is the completed transmitter.

Step 6: Using the Circuit

Now that the transmitter is built, you can use it. Simply connect an audio source (such as a CD player) to the mono jack and turn on the laser. The modulations of current produced by the audio device causes the laser to modulate accordingly. It will get slightly dimmer and brighter, depending on the music. However, this is very difficult to detect by the human eye, and it isn't particularly useful. In order to make the circuit useful, we need build a reciever.

Step 7: Building the Reciever and Using the Device

The reciever is the easiest part. Connect your second mono jack to the solar resistor and battery. You can even place it on the same breadboard, as I have. Just make sure you keep the circuits seperate.

Use:
As stated before, connect an audio source to the first mono jack (the one connected to the laser) and turn the laser on. Connect the other jack to a reciever (such as an amp or the mic. port of your computer) and aim the laser at the solar resistor. The light modulation of the laser are reversed on the reciever and converted back into sound.

Step 8: Alternate Construction and Theory

Instead of using a battery and solar resistor, you could just use a small solar panel. However, these are more expensive and tend to break more easily.

Theory: It may be possible to bounce the laser of glass behind which a conversation is occuring (such as a window) and pick up the sounds on the reciever, but I've yet to test it. Please let me know if anyone has tried this or has a better way to do this.