Uncompressed sound for editing
WAV stores audio without compression, so editing, cutting and applying effects add no further loss. Audio editors, video editors, speech to text services and many hardware devices accept WAV without question. The converter decodes the audio of the MP4 and writes it as 16 bit PCM at the original sample rate.
WAV files are large, about 10 MB per minute of stereo audio. For sharing, choose M4A instead, which keeps the original quality at a fraction of the size.
Long recordings welcome
Lectures, meetings and interviews of several hours work too, because the file is processed on your device without upload.
About MP4
MP4 videos usually carry AAC audio, which can be saved as M4A without re-encoding.
About WAV
WAV is uncompressed audio. Files are large, but every editor and player opens them and there is no further quality loss.