[updated 2 August]
Sam will play Mr Song, River Song's future husband in Doctor Who: The Diary of River Song (Big Finish via Doctor Who TV). This Eighth Doctor series, starring Alex Kingston as River Song, will be released in January 2016. Big Finish tweeted a picture of Alex and Sam when they announced the production.
He has featured in other Big Finish productions - The Vengeance of Morbius (an Eighth Doctor story) and Mask of Tragedy (a Seventh Doctor story).