How to mix waves

[JSoftware]

Just sum them

If you fear it might overflow then adjust gain in the inputs before mixing

(* Pseudo Pascal *)
  FUNCTION MixWaves (Wave1, Wave2: ARRAY OF BYTE): ARRAY OF BITE
  BEGIN
    FOR B := 1 TO Length (Wave1) DO
      Result[B] := Wave1[B] + Wave2[B];
  END;