There's a trick you can use to isolate the vocals in a song that has been mixed. In a multi-track audio-editing application, import the complete song onto one track. Import the instrumental version of the same song onto another track. Invert the waveform on the instrumental track. Play the tracks together and the instrumentals on the complete song track are canceled out. To accomplish this trick you need an audio-editing application equipped with a phase invert tool. Unfortunately such a tool isn't available in the current versions of GarageBand. Therefore you can only isolate vocals in GarageBand on songs that haven't yet been mixed.
Click the "Solo" button (the button with the headphone icon) next to the track you recorded the vocals on. This will mute every other track, except for the vocals, during playback.
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Select the track you recorded the vocals on. Press "S." This is another way to solo a specific track.
Click the "Mute" button (the button with the speaker icon) next to each track that the vocals are not recorded on. This is the long way of muting every other track, except for the vocals, during playback.