Kagura refers to the songs and dances played when worshiping gods in Japanese Shinto. It literally means the music of gods, so there is no doubt that it should be read as music, but most of them People just pronounce it as happy music because of convenience

Another very important point is that kagura is a proper noun, so the pronunciation of the last word is ra and has nothing to do with the Chinese pronunciation of le

As for the pronunciation of the two words music and happiness, music is gaku and happiness is raku. The difference between the two is only voiced and unvoiced. This has nothing to do with the pronunciation of the proper noun "神楽"