Hey Boojoo007,
Download the file that is attached to part one. Then load in your sound file and it should work. I just watched the tut and did exactly that, with r14.
If it goes in both direction, then the “Null-object/Cube” combination might be not as suggested. (As an alternative, you could make the cube editable and set the axis to the suggested side. With the Axis tool or with the Axis Center option.)
In any case, the idea is, that the axis is the origin of the scale, if the origin axis of the scale is on one border of the cube (or the parent Null) then it will be scaled only in one direction.
If you can provide a link to your scene file, I will have a look into it.
All the best
Sassi