With the Music App in the browser on your mobile it is streaming music.
Not sure if whether it converts wav to mp3 or another compressed format on the fly, probably not. Best to save your files as mp3 in the first place if you stream a lot.
Perhaps this thread helps to achieve what you are after: Music apps compatibility with nextcloud
Edit: this may also do what you are after: https://ownyourbits.com/2018/08/28/stream-you-music-with-nextcloud-music-and-power-ampache/
There are different programs to transcode music before saving and playing. Just google around.