Open a digital recording program on your computer. If you do not have any such software installed, you can invest in a professional program like Logic, Pro Tools, Audition or Cubase, or you can download a free program that performs many of the same operations (see Resources). If downloading a free program, be sure to choose one that supports multi-tracking, or the ability to layer your song one instrument (or audio file) at a time.
Place your first audio file on your project window. When you drop the file onto the window, a new track (resembling a long horizontal bar) should appear, unless you specifically place the file onto an already-existing track. Your file will appear as a region, or a smaller shaded bar spanning along the track. Drag the file region to the very beginning of the time line, on the far left side of the track.
Create a second track for your vocal recording. Click the "File" or "Track" menu and locate an option that reads "New Track," "New Basic Track" or "New Audio Track" (as opposed to the "MIDI Track" option, which is reserved for digital notation). Select your program's "New Track" option to create your track directly beneath your original audio file.
Set up your microphone for recording. Although you can record using your computer's internal microphone, sound quality will suffer greatly if you do not use a vocal microphone. For the best quality, connect a vocal microphone to a USB audio interface and connect the interface to your computer's USB port. An interface works like an external sound card to which you can connect instruments and mics and feed the signals to your computer for clear, undistorted sound quality. You can also achieve optimum sound quality by connecting your microphone to an XLR-to-USB cable. Avoid connecting the microphone to your computer's "Line-In" port, as it will pick up excessive noise from your computer's internal hardware.
Connect a pair of headphones to the "Line-In" port on your computer. Since you will record while the first audio file plays, you do not want your microphone to pick up the original recording. By using headphones, you can listen to the original recording and cleanly add your own vocals at the same time.
Click the "Record" button on your software window and record your vocals through the microphone as your song plays in real time. Make sure that your newly-created audio track is selected when recording, and place the time line indicator (usually appearing along the top of the window) at the very beginning of the song, on the far left side. When you finish recording the vocals, click the "Stop" button.
Drag your next audio file onto an area of blank screen within your software window, in order to automatically create a new track for the file. Now that you have recorded vocals to accompany your original audio file, your second audio file should accommodate your mix. For example, you may want to use a loop (an industry term for a sound sample) with appropriate sound effects or background music in the same key as your original file. You can download loops free from the Internet (see Resources).
Mix your project. With your three tracks in place, you must take steps to ensure that each part of the song or audio recording fits nicely into the mix. Each track has a volume fader, which you can use to raise or lower the volume as needed until each section neither drowns out the rest of the mix nor fades into the background. Most programs also feature an array of "Effects" features, which you can select to add useful touches such as echo, reverb and compression. Explore the available effects for each track until your mix sounds perfect.