Who knows ho to do that without any addons?

Use audacity, free program. Or try to find an online converter.

Online convertors work just as well for me. I've used the below online sites without issue:

http://media.io/
http://audio.online-convert.com/convert-to-wav

Here use this: http://audio.online-convert.com/convert-to-wav I used it to make my kiai sound.