Rather than have an open ended list of audio nodes playing, might want to start between 8 and 16 audio nodes, and then cycle round them. So when the 9th note is called, the 1st note stops, and that set of nodes switches to frequency / envelope for 9th note.
For this method, the audio nodes would never stop, they would be started, and just continue. The amplitude would be controlled entirely through gains / envelopes.