In the sixth option, "Audio", be sure this option is activated. The point beside this option confirms its activation, if it is not activated, click on it.

In the last option, "Full processing mode", be sure this option is activated. The point beside this option confirms its activation, if it is not activated, click on it.

Now, new options are active. We go in the "Compression" option.

A news window opens "Select audio compression".
We can now regulate our parameters like above :
Choice of the codec on the left of the window : MPEG Layer-3
Choice of the bitrate on the right of the window : 128 kbps
Then validate on the button " OK " in bottom on the right of the window.