Breath sign doesn't work

Yes, but breathing introduces a noticeable break. So that if someone wrote a series of eight quarter notes with a breath after the fourth, that fourth one should sound shorter - say, like an eighth note with an eighth rest. It might even make the next note a little late or have a stronger attack, although that's more an accident and something players might strive to avoid.

In any case, it is important to keep in mind that MuseScore is primarily about *notation*, and that playback is definitely secondary in importance and priority. So as long as a symbol exists and can be positioned, displayed, and printed correctly, it isn't really appropriate to say it "doesn't work". It's more of a feature request to have automatic playback effects for that symbol added. In 2.0, you will be able define playback effects yourself, so it should be possible to get breath marks to sound however you like.