If you want to record a your voice over a piece of music you have two choices in the NCH Software audio suite. MixPad recording and mixing software is the more advanced program, that will allow you to record multiple tracks, and fine tune the mixing of the music and vocal tracks, but you can also try the Paste Mix feature in WavePad audio editor for quickly combining to files.
WavePad doesn't allow you to record directly over another file, highlighting a file and clicking record will end up replacing the selection. Instead, use the Paste Mix feature to combine two audio files into a single track.
Copy the recorded voice over to the clipboard, then select all in the audio file you want to add that voice over to and click Paste Mix on the Edit Tab.This will open a window with options on what to do if your files are not the same length. If the Mix Audio—which is your voice recording on the clipboard—is the shorter of the two files, you can ignore the options and click OK. If the Mix Audio is longer, you need to decide what to do when the audio you are mixing into is finished. When you click OK the two pieces of audio will be combined and when you press Play you will hear both files.