We'll be doing this in two stages, one hard and the other harder. Well, hard on Cecil, anyway.
After Dwayne at Jambalaya tells you that Cecil was the leader of the C Sharps --
What should I say to the manager of Jambalaya Records?
-- ask Cecil about himself and the C Sharps, then about the Ghost. This will get you two new topics; ask him about them repeatedly until he starts to repeat himself.
We now know that the ghost is Isaac Brown, but talking to him about that doesn't get us anywhere other than his agreeing that's his name. Follow the other lead instead.
How do I find Joseph Mitchell?
After you've talked to Mr. Mitchell, come back to Cecil and ask about Isaac's sister. This is clearly a seriously sore spot, and Lauren can't say anything right. We need some Joey in here.
Switch to Joey and have him left-click on/interact with Cecil. This gets Joey's take on the real situation here, and Lauren now has a new conversation option.
As Lauren, talk to Cecil and choose "talk about love" and "press the point." Ask repeatedly about Isaac and his sister until Cecil starts to repeat himself.
How do I get the ghost to move on?