Like the swallows habitually returning to Mission San Juan Capistrano, the annual “United State Of Pop” mashup from San Francisco’s Jordan “DJ Earworm” Roseman has become a beloved annual tradition in some circles. Each year, Roseman takes little snippets of the 50 most popular songs from the previous 12 months and artfully smooshes them together into an ear-pleasing, five-minute audio collage. The latest model is called “United State Of Pop 2015 (50 Shades Of Pop),” and, as always, the mashup is an opportunity to take stock of a year in music and reflect upon the voices that dominated the pop scene over the last 365 days. The fact that they blend together so easily could be attributable either to Roseman’s impeccable skills or to the overall homogeneity of popular music in the new millennium. Perhaps someone will have to do a “United State Of Pop 1983” or “United State Of Pop 1966” to see if songs from the pre-Pro Tools era are so effortlessly and convincingly conflated.