how to open a .DVDproj file. Want to convert to .mp3

You can't open that file on Catalina because it require iDVD to open and iDVD will not work on it. You can open the project package, Control (right) - click on it and select "Show Package Contents" from the contextual menu. The movie file may be inside it but it's iffy.

There's nothing you can do with a iDVD project file except to open it with iDVD. And then the source files used in the project would have to be in the same location that they were in when first put into the project. There's just nothing you can do.

If you had a copy of the burned video DVD made with that particular iDVD project you could rip it with software to extract the move files in it.