If by sound, you mean the whining or whirring sound you typically hear when reversing, then that is on account of the shape of the reverse gear wheel. In a manual gearbox, and thus automated manuals, forward gear wheels are helical-cut, which is to say the teeth mesh at a diagonal angle. This allows the engagement of teeth to be gradual and silent. A helical-cut gear can also handle more load.
The reverse gear typically has straight-cut teeth; this is simpler and cheaper to produce, but causes the meshing of teeth to be more sudden, thus giving rise to the noise you often hear when driving in reverse.
