Question

The interference of two sound waves of slightly different frequencies produces

Answer 1

The interference produces 'beats' ... two new sounds
in addition to the original ones.

The frequencies of the beats are

   -- the sum of the two original frequencies
and
   -- the difference of the two original frequencies .

Technically, there are actually even more new sounds than that.
There are: 
-- beats between the beats, and
-- beats between each beat and each of the original two sounds.
But their amplitudes are so low that they can be detected only with
sensitive equipment, and they can't be heard in the presence of
the two original sounds and the two 'first order' beats.

Answer 2

Beats.

When two sound waves of different frequency approach your ear, the alternating constructive and destructive interference causes the sound to be alternatively soft and loud - a phenomenon which is called "beating" or producing beats. The beat frequency is equal to the absolute value of the difference in frequency of the two waves.