You'd better start getting used to the sound of it: Claudio Abbado and Berlin. Only the fifth permanent conductor in the century-plus history of the Berlin Philharmonic, Abbado stepped up after Herbert von Karajan stepped down in a snit last April (and, in July, dropped dead). Fear not, the world's greatest orchestra is in good hands. Brahms may have been music's Gloomy Gus, but the Second Symphony is its Sunny Jim, and Abbado and the boys (the BPO remains 99.9 percent male) give it a rip-roaring reading. Nice ''Alto Rhapsody,'' too, although you should feel free to sleep through it if you want to. Everybody else does.