Open Audacity and click "Open" to locate your first audio file. Since you will be mixing the voice file and music file together, it does not particularly matter which audio file you open first.
Go to the "Project" tab and click "Import Audio" to add the second audio file to your Audacity project. This will open your remaining audio file — either vocals or music depending on which file you opened in Step 1. You should now have both the voice file and the music file visible in the same Audacity project window.
Click the audio file you wish to edit, if any. The "L - R" sliding bar changes the pan setting. Drag the sliding cursor left or right to alter that audio file's pan setting. The "- +" sliding bar changes the gain of the individual audio file; sliding the cursor to the left softens the playback level, while moving the cursor to the right increases the volume of that audio file track. Repeat for all separate audio file tracks if desired.
Click "Effects" and select one of the options from the drop-down menu if you wish to further alter your original audio tracks. Effect options include Amplify, Echo, Fade In, Fade Out and Reverse. Press the "Play" button to listen to your Audacity project.
Click "File" and select "Export as WAV" or "Export as MP3" once you have finished all of the necessary alterations on the audio files. This will save the newly mixed audio file to any location you choose.