There are a number of ways you can add a voiceover to audio mark an audio file. If you have a mac, you can use GarageBand which comes free with Mac OSX. If you have a PC we found these tools here: http://www.avs4you.com

How to record voice over an audio track using AVS Audio Editor?

Step 1: Download and install AVS Audio Editor 
Step 2: Run AVS Audio Editor and select an audio track for editing 
Step 3: Adjust recording settings 
Step 4: Add the AudioDorks voiceover track
Step 5: Record your voice over the selected audio track