The OKI M6295 is a 4-channel ADPCM sound chip backed by a 256K ROM containing up to 127 instruments (“phrases”), used in many arcade boards.

This is something the MoonSound should be able to emulate fairly straightforwardly. The sample start & stop addresses are stored in ROM, so the OPL4 sample headers can be precomputed. The instrument numbers don’t exceed the 128-instrument cap. The ADPCM can be decoded to 12-bit PCM. The register only controls start/stop, instrument and attenuation.

Datasheet: http://www.ec66.com/market/sheet/MSM6295.pdf