There are many different types of sound file formats. Mixcraft currently supports the Wave (.WAV & .AIF), MP3, OGG, and WMA formats.
In addition, Mixcraft will work with the Window's Audio Compression Manager (ACM) to load compressed Wave files, other than PCM.
If it is a WMA file, make sure that WMA format support is installed. and that they file does not have Digital Rights Management (DRM) on it.