Mp3s are not supported. Wav files need to be uncompressed (e.g. the ones that come with FL studio are compressed). If you're unsure if they are standard Wav files you can try opening in Audacity. Also I'm not 100% sure from your description, but keep in mind you need to use the WT pos knob to scan through the file, your first frame might be silent if the audio file is not well trimmed (begins with silence).
