Music for Listeners, the taste-making indie-music program on Trinity University’s KRTU-FM, celebrates its 25th birthday this weekend. The crew behind Music for Listeners will celebrate with two free concerts, both at the campus’ Laurie Auditorium. The first, scheduled for Saturday, Sept. 14, will include San Antonio’s quirky popsters Buttercup along with Austin indie-rockers Voxtrot and Blushing.