Basically the only thing interesting about the ConcertMaster is that Carmack used it (and that it looks somewhat cool). As for the keyboard itself, it is just a basic OEM membrane keyboard and not that good one even. The speakers are lets say adequate for the time and size.

Interestingly, the speaker part and the keyboard part are completely separate. The "cable" consists of four separate cables (keyboard, power, line out, mic in) in a thin sleeve. Mine was supplied with AT plug on the keyboard cable and Y-adapter that converted PS2 into AT DIN and barell jack for the speaker power. The keyboard label indeed says it is powered by 9V DC, but I guess that never really happened as PS/2 is 5V no matter what various devices say.

Edit: And as for the supper hard to find nowadays part: I suspect that part of the reason is that the keyboard module inside the thing is ridiculously sensitive to even minor spills.