Why is nitric acid added to sodium extract before adding silver nitrate for testing halogens ? 

Sodium extract is boiled with nitric acid to decompose NaCN and Na₂S, if present, otherwise these

NaCN + HNO₃ → NaNO₃ + HCN ↑ ;

Na₂S + 2 HNO₃ → 2 NaNO₃ + H₂S ↑

will react with AgNO₃ and hence will interfere with the test as shown below :

NaCN + AgNO₃ →AgCN (Silver cyanide (White ppt.))+ NaNO₃ ;Na₂S + 2 AgNO₃ →Ag₂S (Silver sulphide (Black ppt.))+ NaNO₃