You can use a headphone jack to provide audio for recording. Since a headphone jack gives a line-level audio source, the recording will not be amplified, so you may need to boost the volume after the recording is done through your mixing tools. Standard 1/4-inch and 1/8-inch headphone jacks provide a stereo signal through a tip, ring and sleeve connection. The stereo output may not be received by all recording devices. The same tip, ring and sleeve connection for a microphone line-in typically only reads one channel of audio.
Things You'll Need
Converter from 1/4 or 1/8 inch headphone output to microphone input
Connect the adapter to the headphone jack. The most common situation will be an 1/8-inch male-to-male adapter so that you can plug the headphone jack on a computer into the microphone jack on a computer. However, if you're using different audio equipment, you might need an adapter from 1/8-inch to 1/4-inch that is used for instrument line-in on most sound boards or from 1/8-inch to XLR with is the microphone input on most sound boards. The recording inputs on a home stereo system will be the red and white RCA connectors.
Plug the mic cable into the adapter and into the recording device. When you do this, you want both the recording device and the playback device to be powered off to prevent any spike in sound during the connection. This is especially important when you are running a live audio recording as the connection will pop over the speakers.
Set the volume for the headphone jack to be 50 percent. Allow the recording device to account for the low volume after the fact. If the volume is too high on the headphone jack it might introduce distortion into the recording. It's always easier to increase the volume later than remove distortion from a recording.
Test the input by playing the audio on the device with the headphone jack. If you have monitors or a speaker attached to your recording device, set the volume level to about 20 percent. Adjust the volume for the recording by increasing or decreasing the gain level of the input, not the master volume. On a sound board, the gain knob is typically the top knob on the row associated with that source. On a computer, the gain is the microphone input level, not the computer's speaker volume.
Stop the device with the headphone jack. Start the recording on the device with the microphone line-in and the re-start the audio on the device with the headphone jack. Don't adjust the volume on the output device or the gain on the recording device until the recording is over.