¿How to achieve a constant volume in playlists?

I have several playlists comprised of selections from various albums that were recorded at different volume levels. When I play a playlist I'd like all of the tunes in that playlist to have the same volume level so that I don't have to constantly manually adjust the volume for each tune ... gain-riding is a drag. Is there an app that can go through a playlist and determine the average volume for the entire playlist and then PRE-adjust each tune's volume up or down relative to that average volume so that all the tunes in the playlist sound about the same volume as they play? On-line radio stations (like Radio Paradise) present their 'musics' at constant volumes so they must have some sort of tool or app to achieve that. Does anyone know what that tool or app is? Thank you ...