The basis for any sort of Japanese soup dish is going to be dashi stock, a stock made of fish and seaweed. It has a much lighter flavor than chicken broth, so you might not have identified it easily. You can buy it in a powder form for convenience, and it can be sprinkled into other liquids rather than reconstituted. 
 
[This recipe][1] is for soba with a dipping sauce, but it'll give you a good idea of the flavor profile you're looking for: Dashi stock plus [kaeshi][2], which is made of soy sauce, mirin, and sugar (and simmered so it's not strongly alcoholic). 



  [1]: http://www.justhungry.com/basics-cold-soba-noodles-dipping-sauce
  [2]: http://www.justhungry.com/basics-kaeshi-soba-and-udon-noodle-soup-or-sauce-base