Hi Sunny:

Even with a simple "Editor" such as the free version of WavePad... you can display the song-file (WAV or MP3) and highlight individual portions of the song (or melody) and increase or decrease the sound in that segment by a few percentages without distorting the rest of the work.

This method is a bit tedious but will allow you to complete the volume edits, save it to a different file (or overwrite the existing file) and then test it by listening to the final result. If more edits are needed, simply repeat the process. In time, it becomes a bit easier but is still tedious.

I don't do multi-track recording. Instead, I record the vocal and the keyboard song performance into one file, then I use the above stated method to get the little increases or decreases in volume. One thing for certain, I get lots of practice this way.

Best of luck, ----Dave