Unlocking bonus episodes only depends on your rank. There is a total of 3 unlockable bonus episodes. Once you unlock them on one difficult setting, the are open to all others.

Unlockable - How to Unlock
Bonus Episode 1 - Cher - Believe Advance to 4th Rank - Captain of Soul
Bonus Episode 2 - Jackson 5 - ABC Advance to 6th Rank - King of the Beat
Bonus Episode 3 - Destiny's Child - Survivor Advance to 9th Rank - Legendary Agent