Bob Dylan's music was the soundtrack for the counterculture of 1960s America. Now it has become a selling point for an unusual bond offering being marketed to institutional investors and wealthy individuals. A privately held Nashville, Tenn., company is preparing a $300 million bond backed by the cut it receives as a middleman between music companies and songwriters and the outlets that broadcast their music. The company, Sesac Inc., has the exclusive rights to the public broadcast or performance of the music of Mr. Dylan, pop singer Neil Diamond, Canadian rock band Rush and jazz singer Cassandra Wilson.