Download and install the free Audacity audio editor for your operating system
Download and install the FFMPEG plugin, a library to import and export various media formats such as M4A, WMA and MP4.
Audio recording steps (that do the job for me):
- import the sound track(s) first, then record the voice track (below)
- monitor the playback level and reduce the Gain level (e.g. -3.0dB) if appropriate
- select mono microphone recording
- remove the background noise with the Noise Reduction effect plug-in
- compress the voice recording with the Compressor effect plug-in
- use the Auto Duck effect plug-in to reduce background track level
on-line Tutorials
Podcasting with Audacity