Tanks, but i don't understand your code ;D
What I tried now:
I put
extern prog_char SOUND_P[];
in my often included System.h header file, and put
prog_char SOUND_P[] PROGMEM = "Sound.wav";
In one source file.
So it compiles fine, but when I call
char buffer[13];
strcpy_P(buffer, (char *)pgm_read_word(&SOUND_P));
My Arduino just restarts 