Melody and accompaniment audio signals are received and processed to identify one or more harmony notes and a harmony signal is produced based on the one or more harmony notes. Typically the melody note is identified and a spectrum of the accompaniment audio signal and is obtained, and one or more harmony notes are identified based on the melody note and the accompaniment spectrum. The melody, and accompaniment signals can be processed in real-time for combination with the harmony signal in an audio performance. In some examples, audio signals are processed and harmonies generated for subsequent performance based on, for example, MIDI files generated from the audio signals.