Here, i found this info.

Q.  How come Japanese people slurp Japanese and Chinese noodles but not spaghetti?


A.  Jens Jenson was kind enough to write and inform us that:
   1) Noodles (SOBA and RAMEN) should be eaten while VERY hot in order to get the full flavour. When slurping, Japanese
    people take in a lot of air to cool the noodles just before swallowing them. This makes the famous"slurping sound".
   2) Soba noodles in cold broth and nothing else, is considered to be such a simple taste, that in order to get the full experience,
    one should use all ones senses while eating them. Hence the slurping sound will "activate" your hearing and make the
    flavour come out. (Tasting, smelling, watching, feeling should be obvious..)