Manipulate[If [b == 440, Text["No beats"],
  Grid[{
    {Abs[440 - b] "Hz = Beat frequency"},
    {Abs[440 - b]*60 "Beats per minute" },
    {Play[{Cos[2 Pi  440*x], Cos[2 Pi b*x]}, {x, 0, t}]},
    {Plot[{(Cos[2 Pi *440*x] + Cos[2 Pi* b*x]), 
       2 Abs[Cos[Pi *Abs[440 - b]*x]]}, {x, 0, 5/Abs[440 - b]}, 
      AxesLabel -> {"Seconds", Null},
      PlotStyle -> {{Red, Thin}, {Blue, Thick}}
              ]},
    {Text["Red = Graph of the combined sound"]},
    {Text["Blue = Graph of the beats heard"]}
         }]],
 Style["Frequency 1 = 440Hz", 12, Bold],
 {{b, 442, "Frequency 2 (in Hz)"}, ControlType -> InputField},
 {{t, 20, "Duration (in seconds)"}, ControlType -> InputField}]