This seems OP, so I imagine it doesn't happen. However, when the Bard is knocked out his song tokens remain on his song cards. Concentration states "At the start of your turn, if both of your song tokens are on different Class cards, you recover 1 damage and 1 fatigue." This would be a passive ability, but the trigger is satisfied whether he is knocked out or not. So does he get to heal automatically (auto-reviving)? Or does this only trigger when he is not Knocked Out?

Sadgit's wonderful CRRG covers this.

Under the section for knocked out heroes, there is this point:

A knocked out hero may not:

• use abilities or benefit from passive abilities unless explicitly allowed.

Since Concentration does not explicitly state that it can be used while knocked out, it cannot be used by him until he is revived.