He was a friend and advisor of the future Avignon Pope Benedict XIII, and member of the Avignon curia. In 1396 he was an envoy to the Bishop of Valencia to get Spanish support. Benedict had appointed four cardinals, and on his death, three of them, on 10 June 1423, elected Sanchez Muñoz Pope. The fourth, Jean Carrier, absent at the time, declared the election invalid, and elected his own antipope in turn, who took the name Benedict XIV. Consequently Jean Carrier was excommunicated by Clement VIII.
Clement VIII's fate was bound up with the ambitions of Alfonso V of Aragon. Alfonso wished to negotiate for Naples, and so gave Clement support: his queen Maria of Castile, and the Aragonese bishops supported Martin. In the summer of 1423 Alfonso persuaded the Republic of Siena to acknowledge Clement VIII, thus securing recognition for the pope of the Avignon line in the very city, Pavia, which was part of the Republic of Siena, where the Roman pope Martin V had convened an ecumencial council of the Church.
However, when King Alfonso had achieved his political goals, he sent a delegation in 1428 (headed by Alfonso de Borgia, the future Callixtus III), to persuade Clement to recognise Martin. Clement's abdication on 26 July 1429 was confirmed in mid-August. Clement had to make a penitential submission in forma to Martin V, and when this was done Martin granted Sanchez Muñoz a bishopric. Sanchez Muñoz died on 28 December, 1446.