Audacity is a free digital audio recording and editing application for the Windows, Mac OS X and Linux platforms. It is often used to create what are colloquially known as "acapellas" -- vocal tracks isolated, or "ripped," from complete songs. There are a few methods for achieving this, but Audacity offers some tools that make the process relatively simple. However, this method requires that you have a backing instrumentals version of the song you want to isolate vocals from as well as the original song file.
Start Audacity. When it loads, open the "File" menu and choose "Open." Locate the song file from which you will derive an acapella in the file browser pop-up and double-click it. Audacity will load the file on a new track and display its waveform.
Open the "Project" menu and choose "Import Audio." Locate the instrumental version of the song file and double-click it. Audacity will load it on a second stereo track beneath the first.
Zoom in until the waveforms are much more detailed. They will look almost exactly identical, only varying at those points where the vocals are most prominent. You must align the waveforms exactly.
Choose a waveform peak that is visibly identical in both tracks. Click on the "Time-Shift Tool" in the toolbar. Click and hold on one of the tracks to move it left or right in increments. Adjust until the two waveforms are exactly in sync. In most cases, this will not be necessary because the two versions will be timed in precisely the same way, but it is essential that you ensure they are aligned. The resulting acapella will not be usable if they are not.
Choose the "Selection Tool" from the toolbar and highlight the entire instrumental track. Make sure that its entire waveform is encompassed by the selection.
Open the "Effects" menu and choose "Invert." The instrumental track's waveform will be inverted, becoming a mirror image of itself.
Press "Play." With the instrumental track's waveform inverted, it will cancel out the instrumentals on the track with vocals, isolating the vocals from the overall mix. This phenomenon is called "phase cancellation" -- when one of two identical waveforms is inverted, it cancels out the first by being its exact opposite. Only that which the two waveforms do not share is preserved.
Open the "File" menu and choose "Save Project As." Give the acapella a name and click "Save."