I have Canon 550D camera and sometimes when I shoot videos I have noticed that the audio is just too noisy. This is because the camera uses AGC, Audio Gain Control which automatically adjusts the microphone. If you are shooting in quiet place the camera will turn up digital gain of the microphone which will add horrible hiss to the noise. So the only solutions are either install a double wired cable and play some audio constantly to the camera via mobile phone or ipod but I wanted to install MagicLantern which is a firmware hack for Canon cameras. It will also add a huge amount of features to the camera. The actual files will be stored to the memory card and the software will be run from the memory card, so if you remove your SD card the magiclantern is gone. The only downside is that you if you ever format your SD card the magiclantern is gone, but on the other hand it is very easy and quick to install.
Open up your Canon camera and set it to manual (M). Go to the settings and format your SD card.
Go to http://www.magiclantern.fm/ and select nightly build because they don't have stable releases anymore. I have had no issues with the nightly so far.
Select your camera model from the dropdown. The number after your camera version is the firmware version you have to install before running magiclantern.
Make sure your camera firmware is the same as in magiclantern tells. If not head to Canon website and install latest firmware to your camera according to the magiclantern selection.
Unzip magiclantern and upload all files from the zip to the root of your SD card.
Open your camera and go to the firmware version settings page. Press OK to update. Pray. When you see green text saying restart your camera, turn it off and wait 10 seconds. Then turn it on.
Now you should have working magiclantern on your Canon DSLR. However remember that the magiclantern runs from the memory card so if you turn off the camera wait 5 seconds before removing the SD card or as long as it keeps blinking red light. Otherwise you may break your camera.
To access magiclantern settings press trashcan button.