A MT-32 has 512KB of PCM ROM and 64KB of Control ROM. It also has 64KB of RAM for up to 128 user-created custom sound patches. This means that the data to create a custom patch would be 512 bytes per patch, unless the RAM is used for something other than patches. Where is the information for the default patches located? My first guess would be in the Control ROM, which could be copied to RAM when you power the device and can be overwritten for custom patches.

Actually, I don't believe that the MT-32 has 64 Kilobytes of RAM, because based on my reading of the manuals, although my technical ability is limited, the MT-32 doesn't need that much RAM. 64 Kilobits may be closer to the mark. A CM-500 has 32 Kilobytes of RAM, for what I can't fathom.

A CM-32L has 1MB of PCM ROM, the first half of which is the same as the MT-32's to ensure 100% compatibility. The CM-32L also has 33 sound effects samples in the rythym channel. I would guess that the second 512KB of PCM ROM is devoted solely to these samples.