Mark Aitken examines the relationship between photography and sound. Past editions of the show are here. This week: Frank Watson's 'Soundings from the Estuary' is a photo series about the Thames Estuary and a future landscape blighted by rising sea levels. As ever, considering the future requires delving into the past, in this case with Marlow leaving the estuary into the heart of darkness. With music by Robert Wyatt, Gavin Bryars and Dave Lawrence.