Yaarel
🇮🇱 🇺🇦 He-Mage
By RAW, the voice needs to be loud enough to sustain music: "song" or "chant". So, it cant be a whisper. There is a minimum volume. But it can be quiet. You can hear someone humming quietly within 30 feet. Especially so, if someone is on the lookout for a spellcaster. But more than that, or behind closed, doors it becomes inaudible.The existence of counterspell, automatically knowing if a spell is cast within 30 feet of you, and the existence of subtle spell say that you are wrong with this. Verbal and somatic are very noticeable without specific means to avoid like subtle spell and the aberrant mind sorcerer.
Yes it is, but not by volume. At least not by volume low enough to step on the toes of the sorcerer and counterspell like that.