Hi, I'm on a spree to find patents related to old audio hardware. In particular FM synthesis.
Let's see what we find. Bonus points if expired!
I'll start:

Category: FM
- ESS US5578779: Method and integrated circuit for electronic waveform generation of voiced audio tones
This one describes generating FM feedback with a 29 stage feedforward loop, on the spot instead of using sample memory.

- ESS US5581045: Method and integrated circuit for the flexible combination of four operators in sound synthesis
This one describes using a single, flexible FM algorithm akin to a 1D mod matrix, where volume levels control the "algorithm". Pretty sleek.

- Creative US6091269: Method and apparatus for creating different waveforms when synthesizing musical sounds
This one describes generating sine approximation and other derived FM waveforms using fancy logic and some math instead of ROM.
For example, to make sine aprox, you multiply phase by abs triangle made from the phase shifted by half.

- Creative US5900570: Method and apparatus for synthesizing musical sounds by frequency modulation using a filter
This one describes using a 6dB/octave FIR highpass filter to improve the mathematics of Chowning's FM synthesis formula, and make it more Frequency Modulation than Phase Modulation. Or something like that. Amplitude doubles per octave.
The patent suggests implementing it as "out = in[n] - in[n-1]" and storing per-operator past sample.

- Opti US5639979: Mode selection circuitry for use in audio synthesis systems
This describes 2D mod matrix for patching 4 operators to 3 outputs using operations ZERO, GO, BLOCK, to define FM algos.

- Opti US5719345: Frequency modulation system and method for audio synthesis
This one describes FM modulation path that goes through as little log/linear conversions to prevent error accumulation.

- Crystal US5644098: Tone signal generator for producing multioperator tone signals
- Crystal US5665929: Tone signal generator for producing multioperator tone signals using an operator circuit including a waveform generator, a selector and an enveloper
Describes Crystal FM's approach to OPL3 emulation and feedback/noise using wavetable dark magic.

Category: Wavetable
- AMD US6058066: Enhanced register array accessible by both a system microprocessor and a wavetable audio synthesizer
TODO

- Creative US5928342: Audio effects processor integrated on a single chip with a multiport memory onto which multiple asynchronous digital sound samples can be concurrently loaded
TODO