The Covox Soundmaster was a little known sound card that was utilized by a few games. However, it is the only sound card that SimCity supports, but only for sound effects. Other games, like Ultima VI and Bad Blood, support only it for music synthesis. I am sure there is a game out there that supports it for both.

There are two primary considerations in emulating the Soundmaster.

I. Music Synthesis
A. The Soundmaster uses an AY8930, a more powerful version of the much more widely used AY-3-8910 chip. The AY8930 has two modes, one of which is equilavent to the AY-3-8910's capabilities, the second improves on those capabilities. The AY-3-8910 has been emulated in MAME, for Atari ST emulators, for some Apple II emulators, MSX emulators, Spectrum emulators, etc. so there is a start. The advanced mode has not been emulated, and someone would need to tackle that. Fortunately, it would not be a terrific challenge as most of the advances simply add more bits to pre-existing functions. See the datasheet for the chip for further information.
B. The Soundmaster could use I/O ports 220, 240, 280, 2C0, with the default at 220. The AY8930 is somewhat odd in that it has a multiplexed address/data bus and a control bus for decoding. This implies that only two I/O ports are needed, one for the control bus and another for the address/data bus. The tiny photo available to me does not suggest that Covox used a different approach. A quick check of the I/O ports would confirm this.
C. The AY8930 needs a clock signal as a reference for it to work. I cannot tell whether the Covox uses a unique clock crystal mounted on the board or the reference 14.31818MHz signal on the ISA bus. Without an available board, it is impossible to know for sure. Even it if does use the reference clock, that signal must be divided before it reaches the chip because the AY8930 can only use up to a 2MHz clock. Effectively, you can use clock dividers from 8 to 15, eight choices to play with.

II. DAC
A. Programmed I/O is the first type of DAC programing supported by the Soundmaster. If this is equilavent to the Speech Thing, then the Covox must have the basics of a parallel port interface on the card. The card would have to use one of the parallel port I/O selections, most likely 378 to support the Speech Thing, which I have read it can do without a TSR driver. However, if this is something else entirely, then I have no idea how it works.
B. DMA programming is the second type of DAC programming supported by the Soundmaster. As the Soundmaster is an 8-bit card, it is limited to 8-bit DMA transfers. It supports IRQs 3-7 and DMA channels 1,3 (IRQ7, DMA1 defaults) for this purpose. The pictures I have seen of the Covox do not show any custom DSP chip, but Covox could have used smaller PAL or GAL chips to achieve this. Again, I have no further information about this.