Michael Jackson's former doctor is threatening to drop a bombshell on the court proceedings for the singer's wrongful death case if he's asked to testify at trial.

TMZ.com posted audio of a voicemail message Conrad Murray recently left for a friend.  In the  message, the cardiologist warns that he'll reveal a big secret about Michael Jackson if AEG Live and the Jackson Family continue to ignore his requests to leave him out of the trial. 

Murray is serving time behind bars for involuntary manslaughter in connection to the singer's 2009 drug overdose death.  He was previously asked to  testify in Katherine Jackson's lawsuit against AEG Live but pled the Fifth Amendment to get out of it.