I created below tempo change code using following link: https://sourceforge.net/p/sox/code/ci/master/tree/src/example0.c
#include "sox.h"
#include <stdlib.h>
#include <stdio.h>
#include <assert.h>

int main(int argc, char * argv[])
{
static sox_format_t * in, * out; /* input and output files */
sox_effects_chain_t * chain;
sox_effect_t * e;
char * args[10];
assert(argc == 3);
/* All libSoX applications must start by initialising the SoX library */
assert(sox_init() == SOX_SUCCESS);
/* Open the input file (with default parameters) */
assert(in = sox_open_read(argv[1], NULL, NULL, NULL));
/* Open the output file; we must specify the output signal characteristics.
* Since we are using only simple effects, they are the same as the input
* file characteristics */
assert(out = sox_open_write(argv[2], &in->signal, NULL, NULL, NULL, NULL));
/* Create an effects chain; some effects need to know about the input
* or output file encoding so we provide that information here */
chain = sox_create_effects_chain(&in->encoding, &out->encoding);
/* The first effect in the effect chain must be something that can source
* samples; in this case, we use the built-in handler that inputs
* data from an audio file */
e = sox_create_effect(sox_find_effect("input"));
args[0] = (char *)in, assert(sox_effect_options(e, 1, args) == SOX_SUCCESS);
/* This becomes the first `effect' in the chain */
assert(sox_add_effect(chain, e, &in->signal, &in->signal) == SOX_SUCCESS);
free(e);
/*tempo effect*/
e = sox_create_effect(sox_find_effect("tempo"));
args[0] = "1.5", assert(sox_effect_options(e, 1, args) == SOX_SUCCESS);
assert(sox_add_effect(chain, e, &in->signal, &in->signal) == SOX_SUCCESS);
free(e);

/* The last effect in the effect chain must be something that only consumes
* samples; in this case, we use the built-in handler that outputs
* data to an audio file */
e = sox_create_effect(sox_find_effect("output"));
args[0] = (char *)out, assert(sox_effect_options(e, 1, args) == SOX_SUCCESS);
assert(sox_add_effect(chain, e, &in->signal, &in->signal) == SOX_SUCCESS);
free(e);
/* Flow samples through the effects processing chain until EOF is reached */
sox_flow_effects(chain, NULL, NULL);
/* All done; tidy up: */
sox_delete_effects_chain(chain);
sox_close(out);
sox_close(in);
sox_quit();
return 0;
}

For tempo value < 1 I am getting correct resulting output file but for tempo value > 1 I am getting in proper  output file in  which  some of voice samples are missing.How to solve this problem?

thanks