The present invention provides a system and method for generating an output file including steps of generating audio or MIDI content blocks from one or more musical performances; receiving an input file having audio or MIDI music content; determining a chord structure and tempo for the input file; generating a harmonic chord map from the chord structure and tempo for the input file; using the harmonic chord map to automatically select content blocks, and generating the output file by combining the selected content blocks and the input file. The invention enables the creation of unique and new musical accompaniments by re-purposing audio or MIDI content from back catalogs and/or out-takes of musical works. The new arrangement may be provided in multiple music styles, genres, or moods and may contain performances from multiple musical instruments, which may be pre-recorded from live instrument performances and/or of MIDI generated musical content.