You can get a City Tour of Milan Audio Guide that covers 130+ points of interest in the city, plus high-quality visuals, descriptions, and audio commentaries by local experts. The Milan Duomo is covered as part of this great-value audio guide!
Alternatively, the official app DUOMO MILANO, created by Veneranda Fabbrica del Duomo di Milano in collaboration with D’Uva, provides smartphone access to all the information you'll need to visit the Cathedral complex. For an extra fee, you can download complete multimedia tours for the Cathedral, terraces, and museum.