While perusing Street Magic and generating characters for a game I'm running this weekend, I came across the Sound elemental effect:

Indirect combat spells don't behave exactly like the elements they're made up of - lightning spells will strike anything you want them to regardless of grounding, for instance. It could just be that the rules aren't detailed for the sake of brevity, but it could also be because magical elemental spells are made up of magical stuff, which is a lot like the element used but not the element itself.

Since it's never really specified, I think it's up to the GM how realistic the elemental spells are in terms of physical laws. I think it's a matter of game balance, either way. If elemental spells behave realistically, they have some nasty tricks but serious disadvantages. Realistic magical fire goes out in a swimming pool, but if you cast a realistic wind spell on it, you can fan it up to become a cataclysmic inferno without trying to resist the drain of casting a giant area fire spell. Unrealistic fire would burn someone under water since it's magical, but it also wouldn't be affected by feeding it oxygen, since it isn't real fire. Realistic lightning bolts could zap everyone in the entire swimming pool, but if someone wore thick rubber boots, the lightning would not hit them at all and instead would arc into something nearby that's grounded. Unrealistic lightning hits whoever you want, but you can't use water or metal to transform a touch or single target spell into an area attack...

So, does sound make a sound? That's up to the GM and how much realism you want. Realistically, sound must make a sound, though it could be beyond human hearing range, requiring high or low frequency scanners to detect it. Unrealistic sound just acts like sound in how it kills stuff, it's really magic and there's no reason why you'd necessarily be able to hear it.

ixombie is basically right about that, though I wouldn't necessarily limit it to those extremes. A spell's element could behave realistically in certain ways and unrealistically in others. A single target sound spell, for example, could be a loud/high-pitched/low-pitched noise that moves basically in a line toward the target, rather than as a shockwave in all directions. If the sound is audible, then it might be heard by others, but only faintly.

Basically, it could be the real element fundamentally, but with it's behavior limited or altered slightly by the magic.

Also, a possible interpretation of the sound element would be intense vibration. So, the target is simply made to vibrate powerfully. If this is the case, you don't really need to suspend your disbelief about the behavior of the element much at all.