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,