French composer and choirmaster. He succeeded Hughes de Caen as director of the Ancien Chapitre at Notre Dame [...] 1533. Mathieu and Valentin Sohier are often considered to be the same man [...]. Compelling reasons suggest that they are in fact two different composers. Attaingnant, the main printer of works attributed to Sohier, often distinguished between them by adding the appropriate initial 'M' or 'V', even in books that identify all other composers only by surname. [...]
Source: Lawrence F. Bernstein: Mathieu Sohier quoted from Grove Music Online accessed 11 November 2010, Complete article (requires subscription).