VOCALOID4 Editor can import (load) WAV files (in STEREO or MONO/WAV format), but cannot import other audio files (such as mp3, aac, and AIFF files).
Convert the audio file to WAV format with 16-bit resolution at a sampling frequency of 44100, 48000, or 96000 Hz before importing the file.
* You cannot convert files into WAV format in VOCALOID4 Editor. To import a file, use a drag-and-drop operation to paste the file on a WAV (STEREO) track, or under the "File" menu, select "Import" and then "Wave(W)".
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