It’s actually in and working, but the feature wasn’t publicly documented. Here’s how it currently works:
Under [Your Name]\Documents\Guild Wars Gw2Dev folder, you’ll see that GW2 creates a Music folder next to the Screens folder (where screenshots go). All you need to do to get your external music collection playing in the game is to create a playlist of the appropriate name and type in this folder.
Supported playlist formats are .wpl, .m3u, .pls, .asx, and .wax.
Supported audio types are everything FMOD supports by default, which includes .mp3, .ogg, .flac, .wav. aiff, and a bunch of old-school music formats, like .mid or .it files.
There are eight specific playlists the game recognizes, and these are designated simply by the playlist’s file name. They are:
Hopefully these are somewhat self-explanatory in terms of when you might expect to hear them play.
So, for example, if you create a playlist called Ambient.m3u, anything in that playlist will be heard instead of the standard in-game ambient music (the general music you hear when running around in the world).
Each internal playlist can designate a primary and secondary playlist to look for. Most of the playlists will fall back to Ambient if they do not exist. This means that if you’d like to simply replace the majority of the music in the game, you can just create an Ambient playlist, or maybe just Ambient and Battle.
However, if you really want to customize your playlist in detail, you can create the more specialized playlists such as NightTime or Crafting.
The easiest (and at the moment, the only) way to test this is with a MainMenu playlist, since that will replace the music at the login screen.
Source: Jim Boer, Guild Wars 2 Developer
Wooded Kingdom Moons in Super Mario Odyssey