I have an ATMega328p-based board I created for another project, which streams 44 KHz WAV files from an SD card and pipes them through a DAC, amp, and then a small speaker. By stripping out some stuff it doesn't need (like an on-board high precision RTC) and changing it to fit whatever DC power is readily available inside the Altair case, I could easily redesign it to be an inexpensive audio module that plays disk drive WAV samples.
I only have one question: what signal is available on the Altair Clone that can communicate information about the disk drive? Is it a one-pin solution, or can something more complicated (two-wire) be available?