There are many different types of sound file formats. Mixcraft currently supports the following formats:
In addition, Mixcraft will work with the Window's Audio Compression Manager (ACM) to load compressed Wave files, other than PCM.
You can also load MIDI files into Mixcraft, of course.
If it is a WMA file, make sure that WMA format support is installed and that the file does not have Digital Rights Management (DRM) on it.