Do you want mix voice track with background music track. Here is step to mix two tracks together in audacity (please downloadaudacity).
Step 1: Import first track in the audacity using following options File->Import->Audio.
Step 2: Import second track in the audacity using following options File->Import->Audio.
Step 3: Choose the Time Shift tool and adjust the position of one track or the other until they’re synchronized the way you want them. You can even move tracks around while they’re playing.
Step 4: Export tracks as mp3 using following option File->Export. You will get output as mix of two tracks.