On the original bob it, the expert setting is the second level of difficultly. It has the edition of sounds instead of just spoken commands.
How to unlock
One unlocks expert by getting a high score of 100 on the first difficultly level (novice).
The new sounds
The sounds will give out a banging sound for the button, a longish sound for the pull it and a similar sound it outputs for twist it.
Remember all the sounds are similar to the sound it outputs.
Where to go from here
Once you get 100 high score in the expert setting, you can unlock Master difficultly.