Video games and music have influenced each other since the beginning of the consumer video game era. In particular the chiptune genre of music uses sounds from 8-bit video games; these sounds have even found their way into contemporary popular music. However, in this genre, game sounds are arranged using conventional musical interfaces, meaning the games themselves (their algorithms, design and interactivity) play no role in the creation of the music. This paper describes a new way of creating music with 8-bit games, by reverse engineering and augmenting them with run-time scripts. A new API, Emstrument, is presented which allows these scripts to send MIDI to music production software. The end result is game-derived musical interfaces any computer musician can use with their existing workflow. This enhances prior work in repurposing games as musical interfaces by allowing musicians to use the original games instead of having to build new versions with added musical capabilities. Several examples of both new musical instruments and dynamic interactive musical compositions using Emstrument are presented, using iconic games from the 8-bit era.