Open GarageBand and create a new session. Do this by clicking "File" then "New" and selecting "New Music Project."
Name the project "GarageBand Mix" and click "Create."
Add audio files to mix by dragging them into the GarageBand edit window.
Adjust the volume of each track by moving the volume slider. Move the slider to the left to reduce the volume of the track and to the right to increase the volume of the track.
Adjust the pan knob of each track to pan it left, right or center. In music mixes, the vocal, bass guitar, kick drum and snare drum are typically panned center. Background vocals, guitars, keyboards and tom-toms are typically panned left or right.
Click a track to highlight it, select the "View/Hide the Track Info" button and click the "Details" arrow to access the options for adding effects to an audio track. Choose from a variety of effects, ranging from compression to EQ to reverb to flange to distortion and more. From each of these effects, choose from a variety of presets or simply manually adjust each control to achieve the sound you are after.
Export the mix from GarageBand when you have finished adjusting your mix. To do this, click "Share," select "Export Song to Disk," name the GarageBand mix and click "Save." This will create a single stereo audio file from the combined audio files in your GarageBand mix.