If you adjust the slider of the "enemy monsters", in the music and sound section of the menu, to off, XP orbs will not play a sound when you pick them up.
How to replicate:
1. Create new world.
2. Press esc and select "Music and Sound".
3. Adjust the slider of the "Enemy Monsters" to "off".
4. Type "/summon XPOrb ~ ~ ~" (without quotes).
5. Notice how the sound of picking up the XP Orb doesn't play.