I also imported everything but it doesn’t work for me when I try my code. Is there anything you are doing differently? I have been trying to get mine to show up for quite a bit of time now and I’ve tried everything by now.
I don’t know what I’d try next based on the class not found error. It seems like the the PApplet.main("Sketch"); line isn’t correct. I’d go back to this article or some of the minim examples and see if you can get them running before going back to tackling your own code.
For you image you shared, you have the mp3 file in the folder data, how do you do that? Because I had my files in with the classes.
Right click on project > new > folder. Then right click on data folder > import > filesystem > find folder with your mp3s and select them. Finish and they should show up in the data folder.