What are 'Musical Hallucinations'?

Dr Victoria Williamson appeared on Radio 4 on New Year's Eve talking about musical hallucinations for the popular science show "All in the Mind". Who experiences musical hallucinations and why? Is there a link between musical hallucinations What are 'Musical Hallucinations'? and 'earworms', the more common experience of having a tune stuck in the head? And finally, what can musical hallucinations potentially tell us about the nature of our minds? The link is available here: http://www.bbc.co.uk/programmes/b04vklfv