Do not convert Opus audio files to WAV

[albertvillanova]

As suggested by @severo, we could avoid the conversion of Opus audio files to WAV: #2811 (comment)

Indeed, according to https://developer.mozilla.org/en-US/docs/Web/Media/Formats/Audio_codecs, it's a well-supported codec among browsers (see "Browser compatibility" in the Opus table).

Beware:

Safari supports Opus in the <audio> element only when packaged in a CAF file, and only on macOS High Sierra (10.13) or iOS 11.

Overall:

Opus is a good all-around audio codec for use in your web applications, and can be used for any audio tasks you have in mind.

[Airop]

Hey, I would like to work on that!