Comment 2 for bug 1380576

gmusicbrowser is a library-based player, wav files typically do not have metadata, so they do not really belong in a library and thus support for wav files was never intended.
I might add some support for wav files in the future, depending on changes that would make this easy, but it's not really planned.

If you want to add wav files to your library, I recommend converting them to flac.
If you just want to play some random file on your system, I'm afraid you'll have to use another player. There is a lot of odd sound file formats that will probably never be supported, gmb's intent is not to play every file format, but to make the best use of the metadata of your library.