Actually the packages provides its own copy of awe_voice.h, for non-linux systems. It should be fine to use that, as the attached patch does (it does compile, and since that's almost the same header, the result is exactly the same).
Samuel
diff -ur sdl-mixer1.2-1.2.6.orig/native_midi_gpl/playmidi.h sdl-mixer1.2-1.2.6/native_midi_gpl/playmidi.h --- sdl-mixer1.2-1.2.6.orig/native_midi_gpl/playmidi.h 2002-02-13 20:07:39.000000000 +0100 +++ sdl-mixer1.2-1.2.6/native_midi_gpl/playmidi.h 2007-07-25 01:30:22.000000000 +0200 @@ -52,11 +52,7 @@ #include <stdlib.h> #include <sys/soundcard.h> #include <sys/ioctl.h> -#ifdef linux -#include <linux/awe_voice.h> -#else #include <awe_voice.h> -#endif struct chanstate { int program;