For all my previous games, I've employed MIDI format music-- sometimes to my composer's chagrin. I'm just not what you'd call a technical person-- one of the prime reasons that I use the Game Maker program!-- and my previous attempts to implement other formats (like MP3 or ogg) or to use .DLLs have fallen woefully short.
At least, they did until I came across the Sin Bass DLL on the Game Maker Community forums. This .DLL, created by Sindarin, and its accompanying set of scripts, makes adding high-quality music, and applying all sorts of nice bells-and-whistles, such as fades and pitch adjustments, so simple that even Tom Russell can do it. My composers, both for Seq.Breaker and Side Saddle 3, are as excited as all get out.
If you're still using MIDI music, and you'd like to step up your game, Sin Bass is a great and accessible way to do it.