Does one who approves of musical instruments found in films become an infidel, given the possibility that such instruments were previously accompanied by blasphemous lyrics in a song, and what is the ruling on one who approves of disbelief while not knowing it is disbelief, but rather knows it to be a sin?

It appears that the questioner is suffering from whispers of disbelief (kufr), and the cure for them is to turn away from them and ignore them. Being pleased with musical instruments in films, which are suspected of containing blasphemous words, does not lead to disbelief (kufr). However, it is forbidden to listen to such musical instruments and to watch films if they contain forbidden images. Being pleased with a sin and committing it does not lead to disbelief. Rather, a Muslim commits disbelief only if he commits an act or utterance that necessitates disbelief, while being sane, freely choosing, and intending it.