Phonetics and the IPA 2, Exercise 3: pinching the nose

mama: changed. [m] is nasal, so the velum is lowered and air leaves through
      the nose. With the nose pinched the air is trapped and the sound is
      muffled or stops.
baba: unchanged. [b] is oral: the velum is raised and the nose is closed off
      anyway.
sasa: unchanged. [s] is oral too.

What it tells you: the velum acts as a valve. Raised = oral sounds. Lowered =
nasal sounds (and nasalised vowels). Only the nasal sound depends on the
nose being open.