I've always run it that it makes a wooshing sound (like when a gas stove ignites). Which isn't soundless, but is hardly loud either. The armored guards falling over is still more than likely the primary consideration as to whether anyone hears anything (the same as sleep).
In 5e we have several spells, like Thunderwave, that explicitly call out that they're audible at 300 feet. Fireball has no such rule, and in point of fact the 5e PHB describes it as a low roar, so clearly even in 5e fireballs are not intended to be loud spells.