Michael Jackson reportedly recorded the vocals for his upcoming posthumous album Michael via a series of mobile phone messages before his death.
According to The Sun, the quality of the recordings might be the cause of the controversy over whether the vocals are actually by Jackson or an impersonator.
A source said: "A large chunk of the album came from voicemails. Michael was certain new stuff would get out so he decided to sing lyrics down the phone.
"After Michael's death last year there were question marks over whether the messages would be able to be used.
"Record bosses were worried whether it would come off at all. Luckily it did, but it hasn't been an easy process."
Sony Music reportedly paid around %C2%A35 million for the tracks from a number of people who have claimed co-writing credits on the songs. The label has allegedly also spent a significant sum to analyse the material to ensure that it has not been faked.