Melf's Unicorn Arrow from PHBII. It's longer range (medium), lower level (3rd), untyped damage, more damage, and still ignores SR. It also bull rushes the target, which could be handy for making the target provoke attacks of opportunity.
The "more damage" claim is only true at high levels, though. The Orb spells are mostly 1d6/level, and Melf's Unicorn Arrow is 1d8+8 per horn, with 1 horn + 1 per 3 levels above 5th (max 5 horns). It does more damage than Orb of Sound at caster level 8+, more than Orb of Force at 11+, and more than the other Orbs at 14 and 17+ (it's 1 point shy at 11th). Also, an empowered MUA only takes a 5th-level slot.
The "more damage" claim is only true at high levels, though. The Orb spells are mostly 1d6/level, and Melf's Unicorn Arrow is 1d8+8 per horn, with 1 horn + 1 per 3 levels above 5th (max 5 horns). It does more damage than Orb of Sound at caster level 8+, more than Orb of Force at 11+, and more than the other Orbs at 14 and 17+ (it's 1 point shy at 11th). Also, an empowered MUA only takes a 5th-level slot.