I have been making a recent study, based on the poorest evidence, of what the main features of each particular Covox device marketed for the PC. Here they are:
Covox Speech Thing - External parallel port DAC for digitized sound playback.
Covox Voice Master - External parallel port DAC and ADC for recording voice samples.
Covox Soundmaster - This 8-bit ISA card used the AY8930 chip for music synthesis, two 9-pin Atari-style digital gameports, and an 8-bit DAC utilizing DMA as well as being Speech Thing compatible.
Covox Soundmaster+ - See below for the Soundmaster II for specs, except it does not have any recording capabilities.
Covox Voice Master Key - An 8-bit ISA card with an 8-bit DAC and an 8-bit ADC for recording and playback of speech samples. Speech Thing and external Voice Master compatible. Also utilizes DMA for sampling rates of up to 44,100kHz.
Covox Voice Master Key II - The Voice Master repackaged in a box with a speaker.
Covox Soundmaster II - Replaced AY8930 chip for the Adlib YM3812 chip. Ditched digital joystick ports for a standard analog game/midi port. Has all the capabilities of the Voice Master Key for digitzed sound.
All Soundmasters and the Voice Master Key can be set to the following:
I/O: 220, 240, 280, 2C0 [default 220]
Digital IRQ: 3, 4, 5, 6, 7 [default 7]
DMA: 1, 3 [default 1]
The Soundmaster+ and Soundmaster II can also be set as follows"
Midi IRQ: 2, 3, 5, 7 [default 2]
Adlib I/O: 388, 380 [default 388]
Midi I/O: 330, 338 [default 330]