When is the Euphonium getting its own Musescore_General sound?
As far as I'm concerned, the Euphonium is still using the Tuba sound. The Tuba sound, in it's higher range is very loud and can be heard through an entire orchestra blasting fortissimos.
Sure, there's the option to download a Euphonium soundfont and save that to default but every time you would have to set the Euphonium to that separate sound whenever you make a score. I'm sure there's a workaround for that minor inconvenience but what does become a problem is that the soundfont likely isn't supported by someone on the MuseScore team so any bugs that occur with that sound become the user's problem. In addition, many soundfonts haven't added support for expressive dynamics so I doubt any Euph soundfonts have done so already.
Euphonium may not seem like a standard instrument worth getting its own soundfont yet it is standard within Concert Band, Brass Band, Marching and Drum Core (which is worth mentioning that Baritone Horn is essentially the same sound as the Euphonium), and it appears frequently in Symphony Orchestra music.