XP orbs play multiple instances of the pickup sound (with different pitches) simultaneously. I'm not sure if this is due to there being both minecraft:entity.experience_orb.touch and minecraft:entity.experience_orb.pickup, or from another cause. The result is a rather unpleasant high-pitched discordant sound.

To verify, /summon XPOrb ~3 ~ ~ and listen for more than one XP ding with different pitches.