Focusing on the sound:
* Sizzling soup slurp
* Silently slurping soup
* Smoothly slurping soup
Focusing on the action:
* Savory soup slurping
* Satisfying soup slurping
* Speedy soup slurping
Focusing on the taste:
* Spicy soup slurping
* Sweet soup slurping
* Steaming soup slurping
You can also use alliteration with specific types of soup, like:
* Spicy seafood soup slurping
* Creamy carrot soup slurping
* Hearty lentil soup slurping
Ultimately, the best alliteration for you will depend on the specific context and the type of soup you are describing.