Unimplemented sound systems

Keyboard synthesizers

Roland linear arithmetic (D-50, MT-32, etc): Preliminary support on MAME
NEC uPD933 (Casio phase distortion): Only recently marked "good" on MAME
Yamaha's pre-FM digital chips (YM1019, YM1104, YM1105, IG10771): No reverse-engineering efforts or adequate documentation, extremely obscure, mostly high-length wavetable chips
Curtis CEM3394 and National Semiconductor MM5837 (several analog-subtractive synths, Sente arcade machines): MAME support only recently became mostly complete. Sente games use it more like a 2x SID
Yamaha DX7: Monotimbral, meaning only one unique instrument is active at a time, and 16 voices means 16 instances of only one instrument can be active at a time
STMicro M114S (Amstrad CKX100, several European (mainly Italian) synthesizers and organs, Zaccaria pinballs as Mr. Game): Inadequate emulation and only on PinMAME. Its main selling point is wave morphing, which can already be done in the wavetable tab of the instrument editor.
Yamaha YMW258-F GEW8 (Sega 315-5560 MultiPCM): Almost the same as the PCM part of the YMF278B, and same reason

Atari AMY: 64-voice additive synthesizer with complete docs, but no emulation material
NEC uPD1771c (Super Casette Vision): An MCU with an embedded ROM. Imperfect emulation on MAME
Sega 32X PWM: A pair of DACs with software-mixed audio. The Gameboy Advance is already a precedent for a variable number of software-mixed voices into two hardware outputs
N64 RSP: Same as prior
Yamaha SCSP (Sega Saturn): Too overwhelming to understand or implement
Yamaha YMF278B OPL4: 24-voice CD-quality PCM chip with an embedded OPL3. The fork with it needed lots of improvements and fixes but hasn't been worked on in a few years, and the code needs to be completely redone
Astrocade PSG: Limited capabilities, and too few potential users
Mountain Computer Music System: Barebones 16-voice wavetable synth of 256x256 waves, as a pair of sound cards for the Apple II. Only used for professional music production software, and easily imitated with an ES5503
Signetics 2636: Used in the Century Electronics CVS arcade system and a few small home consoles. CVS games use pulse widths and other quirks
Yamaha YMF288 OPN3/EPSM: Same as the YM2608, but without the ADPCM or CSM. Any importers or converters from FamiStudio or other places should target their EPSM data to a YM2608
OPP: Differences between OPM and OPP are extremely insignificant

Stern BSMT2000: A masked-ROM version of the TMS320C15 DSP, used as a 12-voice sample chip. Needs a ROM to work
Yamaha YMF271: Same reason as SCSP, and also inferior emulation despite being in the VGM spec
Capcom CPS3: Probably the same as QSound, or mostly
Galaxian custom sound and derivatives like in Sigma's "Spiders!" and SNK6502 games: Typical discrete netlist circuits consisting of a 4-bit linear counter and several analog circuits that are hardwired to produce specific SFX. Explained at Plogue's blog post. Pong is the only exception to hardwired netlist circuits not being added, and it was added as a joke. Plogue Chipsounds has a few of these systems (Galaxian, Donkey Kong, Zaxxon, Phoenix)
OKI MSM5205: Planned
Williams-Bally DCS sound system: That's a codec, not an actual custom circuit. The hardware is (initially) an ADSP-2105 processor running an engine that can do up to 4 simultaneous sounds.
Namco C352, Ricoh RF5C400, Taito ZSG-1, IGS ICS2115, Hyper Neo Geo 64, Yamaha AICA, Eolith QS1000, other >24 voice hi-fi PCM chips: Redundant with the ES5506, but the SPU might have features that stand out
SNK Wave: Equivalent to the Commdore PET, but with real registers and the wave is 3-bit instead of 1-bit
"Exidy SFX" or "Exidy SFX + PSG": Netlist circuitry that's equivalent to a 3x PC Speaker, via a Motorola MC6840 PIT for the square waves and a 128-bit Motorola MC14562 shift register for the noise

Konami K054539, Acorn Archimedes VIDC, Macintosh, Gaelco, Brezzasoft Vrender0, Imagetek I5000, : Easily redundant with generic PCM DACs or a YMZ280B, while the Macintosh is specifically redundant with the Amiga Paula and the I5000 is redundant with QSound
Software-based sound DSPs (Atari Jerry, N64, Atari STe, 32x): The GBA is an exception due to having features specific to it, but these are all CPU-driven softmixed audio. The Jerry is said to have "FM, wavetable, PCM" but those are advertised driver functions. The N64 is said to have 100 PCM channels, but that's theoretical and assumes that each output stream takes a maximum of 1% processing load.
Yamaha SMAF chips (MA-series): An entire community was invested in emulation and reverse engineering projects, with their furthest progress being manual interfaces to the chips. No reverse-engineering was done on the official emulation cores
MT32 / LA32, Sound Canvas, other MIDI modules: Furnace is not intended to be a tracker for any MIDI devices, rather being intended for being able to directly drive sound chips. There are multiple issues with implementing devices like these, such as lack of control over what the hardware does, along with requiring copyrighted data and code
PICO-8, Voxatron, Picotron, TIC-80: Lack of control over sound, same as MIDI in this regard