Or they may have been attached to other memeplexes. Beethoven's mind collected these memes and somehow they got connected giving rise to his famous symphony. Now they depend upon each other for continued replication. Of course, the question remains. The answer to this is quite simple.
Memes are essentially sets of instructions that can be followed to produce behavior.
A meme that produces the behavior of whistling the first 4 notes of Beethoven's fifth can be encoded in any of these systems and it will give rise to the same behavior. The best way to think about this is to consider an analogy in the computer world. Imagine that a robot is developed which contains a number of built in programs. One of these programs gives it the ability to write small behavioral routines. Essentially the robot can alter its behavior by writing small programs.