I found one relevant patent from 1995 relating to using DSP for sound synthesis but as there are currently many open platforms and resources available for the creation of this type of synth I likely wont have to worry about any IP conflicts during the development of the product. The major licensing issue comes with applying the USB standard to the device as a license can be multiple thousands of dollars. Thankfully, there is a large community of people online who legally share device licenses that were grandfathered in before this requirement was put in place, so that cost would be mitigated. Some sound effect algorithms might be proprietary but there are many to choose from so those could be avoided. The closest products that currently exist on the market are the Novation Peak (or summit), UDO Super 6, and Waldorf Kyra which are all synths with digital oscillators and some sort of FM capabilities. While they all have their merits and are quite powerful, none have the specific algorithm based FM I would implement and instead only offer simple oscillator to oscillator FM with fixed routing.