I use the term 'VN' because it's more descriptive. Terms like 'eroge' and 'galge' are borderline useless, because they give you little information about the gameplay mechanics of a title. When people search for games, one of the first things they do is narrow the pool down via gameplay mechanics, then they start looking at the details. The term 'visual novel' gives you much knowledge about the mechanics of the game in a two word title, 'eroge' gives you nothing except 'it has erotic content', information you could get from the ratings sticker (Lightning Raidy is an eroge and so is black bible, but one is a rpg and the other is essentially a novel.) Galge games, for example, can have 'visual novel' mechanics or 'dating sim' mechanics, 2 very different types of games. Some people may like stories but hate having to manipulate stats or micromanage day-to-day shit like you would in a dating sim, so the term 'galge' would mean little to them.
So the term 'visual novel' is a relatively accurate description of the gaming mechanics of certain games, which is probably why it's officially used. Just like 'Gears of war' is officially categorised as a 'third person squad shooter', because it accurately describes the gameplay mechanics, not a 'space marine shooter' because that tells you SFA in comparison (yes, I'm referring to you Kitouski )
Concerning the term 'sound novel', it's too close to 'audio book' which means something else entirely.