Original D&D used to be just Fire, Ice, Lightning, Acid and Sonic. But with the onset of 3e-3.5e/4e D&D there came a multitude of new energy types to list after spell names, like Negative, Holy, Unholy, Dark, Light, Steam, etc. These were put into place so the writers had an excuse to make a spell effect that nobody was immune to. Unholy Fireball? Damage is half "fire"/half "unholy!" Which means that a creature with a ring of 50% fire resistance would only be resistant to 25% of the damage instead of just 50%, because half of that is Unholyness. And that's only if the damage separation was even. Sometimes it was 75% one energy type and 25% the other energy type. So if you were 50% fire immune to a spell that onIy dealt 25% of its damage as fire, then you reduced damage by 12.5%. Yeah, it got weird.
However I'm just going to roll with the five original energy types. And my answer is:
Almost nothing is immune to sonic damage simply because people forget to list it in the monster description. One thing sonic really has going for it is that in many editions it does double damage to golems and construct-type monsters. Sometimes its just crystal golems that suffer double damage so no one cares, but whatever. And while the sonic spells aren't that powerful, I like the idea of casting a cone of vibration to shatter weapons and stuff.
Fire and Ice Wizards are cool and easy to visualize, sure. They can
throw ice spikes and fireballs around and everybody is down with it.
However, I like the idea of a Sonic Wizard. He would look exactly like
Dio. And while there aren't that many purely sonic spells to benefit from, I think a Sonic Wizard could
make up for his offensive casting shortcomings with the ability to spout pure Rock n' Roll at any time. His magic Staff would in fact be a Magic Rock Lute and whenever he cast spells there would be a sweet guitar riff and a lightning bolt would hit a volcano off in the distance.