Is openly committing sins, such as watching movies or listening to music, considered an act of disbelief (kufr)? And does this ruling apply to minor and major sins alike?

Openly committing sins is a sin on top of the original sin, and its perpetrator is threatened with deprivation of God's forgiveness, as the Prophet, peace and blessings be upon him, said: "All of my nation are forgiven except those who commit sins openly." There is no difference in this between minor and major sins. Sins vary in their sinfulness, so one who commits a sin secretly is less sinful than one who commits it openly, and one who regrets is less sinful than one who boasts. A Muslim should follow up his sin with repentance, seeking forgiveness, and remorse, not with boasting and open declaration. As for those new to Islam, they are excused for their ignorance of religious rulings, but they should be taught and guided.