I just found out about Mp3 Gain .

Would it work for the songs if a MM Playlist is already created ?

Or do you have to start a New project ?

Hey MegaMix, many of us here use Mp3Gain, it's a great tool! The answer to your question is yes, you can use it on an existing playlist. Just

open up Mp3Gain, add the same files you used on the playlist. Make sure that you don't have MM open while doing this! Hope that helps