Tommy already answered the basic facts, so I would like to put the various assumption a bit into perspective.
The Mac version is entirely mute, save for some random sound samples which seem to be recorded with a microphone.
Which is what the Mac's sound 'system' was made for.
It is said that the single channel 8 bit/22 kHz DAC hardware was included on Steve Jobs' request especially for his "out of this bag" stunt, although Andy Herzfelds recollection tells this being exaggerated, but showing his dedication to the sound system. In addition one of the 6522 timers could be used to mute/unmute the sound to produce simple sounds (including higher than 11 kHz) or create additional effects.
In an interview I recently listened to with the authors, they said that everything was all about saving data in that game for it to fit in a floppy.
Which is half the story. The other parts is scarcity of main memory, CPU power and low floppy speed. The original Mac was a machine that had way more in common with back then home computers than what we think of it now - heck, even a C64 can outrun the original Mac in certain areas - And Shadowgate was designed in 1984/85 when the Macintosh came with 128 KiB of RAM which had to fit OS, video RAM (22 KiB), code and of course all resources (graphics, sound). As a result a lot of loading was needed.
But storage was only a single 400 KiB Floppy drive. A prime goal was to fit everything onto a single floppy - flipping floppies is the best way to kill immersion - so not exactly much space either.
So while Sound can be compressed to some degree, it needs a lot of code, CPU power (an 8 MHz 68k isn't a racer) and even more RAM space. With a Machine that strapped, it's hard to have a buffer for a second of continuous sound (22 KiB). Of course amount of data can be reduced by syntesising sound from data but that again needs lots of CPU and code.
In contrast, the NES version came on a 256 KiB TKROM, ready to execute in place with no need to load from a slow floppy. Even more important, it has 5 sound channels which can be programmed using way less than 22 KiB/s data rate and more important no need to use RAM for that purpose.
But if every little bit mattered so much, how did they fit all those sound samples?
One might have to look at each separat, but I would think the main 'trick' was keeping them short. Karl Roelofs talks a bit about this in a Blog Entry which pretty much summs it up:
So, in order for us to make this work, we had limitations:
- No music (except for credits of course)
- Scant sound effects
- Utilize patterns as much as possible
Don't even a very short such sample take a huge amount of space? WAV files, etc.?
That's exactly the point here.
Compared to any sort of MIDI-like background music which would take much less space to store than the sound samples.
True. But MIDI (or any kind of procedural format) will need some device, hard or software to turn it into a wave form to be outputted. In case of the Mac it would have been main CPU and main RAM. Neither available in large quantities.
Yet the game is entirely silent in terms of music. This doesn't make any sense to me. If the Mac was able to play back realistic sound samples, surely it must have also been able to output some kind of music? The "storage space" argument doesn't seem to hold up at all.
Oh, it does. In any way imaginable.
Historic side note: Apple joined the (International) MIDI Association as early as January 1984 as corporate/manufacturer.