Voicing of voiceless stop after nasal is a sound change in process, documented by Joseph & Arvaniti (2004) etc. in rebetika song of a century ago. The non-nasal pronunciation of ντ μπ γκ is emerging as standard.
An interesting example many are familiar with is the Athens metro, where the voice of the train conscientiously nasalizes most of these stops, but lapses into the emerging standard at Αμπελόκηποι. Contrast Chalandri ND, Doukissis Plakendias ND, Abelokipi B, Evangelismos NG, and Syndagma ND