Does this mean you have installed a more recent Minim version from it's website? If so, have you removed the Minim version that comes with Processing, as the website you link to instructs? Otherwise multiple versions of Minim may cause problems.It's not the library that shipped with Processing. I looked it up and it said you had to use this program to play sound, so that's what I did.
Note by the way, that the (older) Minim library that comes with Processing is perfectly fine for playing sounds and includes the code sample you posted.