Minor compiler warning and inconsistency in example 06-sound-adpcma #13

Hello,

I noticed a small compiler warning in the 06-sound-adpcma example. Here it is:

ngdevkit/local/bin/m68k-neogeo-elf-gcc `pkg-config --cflags ngdevkit` -std=gnu99 -fomit-frame-pointer -g -c main.c -o main.o
main.c:37: warning: "REG_SOUND" redefined
   37 | #define REG_SOUND ((u8*)0x320000)
      |
In file included from ngdevkit/local/m68k-neogeo-elf/include/ngdevkit/neogeo.h:28,
                 from main.c:22:
ngdevkit/local/m68k-neogeo-elf/include/ngdevkit/registers.h:65: note: this is the location of the previous definition
   65 | #define REG_SOUND       ((volatile u8*)0x320000)
      |

It seems likely that this was defined locally to illustrate the core concept of writing to this memory address. However, perhaps the local redefinition should also be volatile? I'm just getting started with Neo Geo development, so this is just an educated guess.

By the way, thank you for putting this development kit together. It's very well designed, and easy for me to follow along and make sense of. The examples that compile everything from the original assets with Makefiles are an enormous help. You have truly provided everything people need to make a complete, functional Neo Geo game in one place. Bravo!