Audioviz produces audio visualizations, and as a by-product also allows for audio to gameplay syncing. Audioviz version 2 now provides FFT data for frequency spectrum display!
Audio files must be OGG format, and included with the project as "Included Files" rather than sound resources (or any other way for GM to pass a path to the DLL). Mono, stereo, and 5.1 are supported.
Usage demos are included in the project, with annotated code.
Demo music by Patrick de Arteaga: https://patrickdearteaga.com/royalty-free-music/