sox_ng wiki - Includes
The include files cascade in this order:
"sox_i.h"
"soxomp.h"
"soxconfig.h"<omp.h>"sox_ng.h"
<limits.h><stdarg.h><stddef.h>"util.h"
<stdio.h><stdlib.h>"soxconfig.h"<sys/types.h><sys/stat.h>"xmalloc.h"
<stddef.h><string.h><strings.h><inttypes.h> or <stdint.h><math.h><assert.h><errno.h><byteswap.h>"effects.h"<ltdl.h>sox_ng.h is included by users of libsox_ng
sox_i.h is included by all sox-internal source files
sox_ng.c is the only source file not to include sox_i.h because it is a user of libsox_ng, but it includes util.h for the usual sox type definitions and macros that replace stricmp etc.
That leaves other source files to include, if they need them,