Program to reverse a given WAV file in C not working

Question

I'm writing a program to reverse wav files in c. I've (seemingly) finished it and looked over it many times to find any logical errors. It compiles into an executable program, and outputs a wav file. But the output file in corrupted/incorrect somewhere and will not play audio. Do y'all see any problems with it? Thank you.

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "wav.h"

int check_format(WAVHEADER header);
int get_block_size(WAVHEADER header);

int main(int argc, char *argv[])
{
    // Ensure proper usage
    if (argc != 3)
    {
        printf("usage: ./reverse input.wav output.wav\n");
        return 1;
    }

    // Open input file for reading

    char *input_name = argv[1];
    char *output_name = argv[2];
    FILE *inptr = fopen(input_name, "r");

    if (inptr == NULL)
    {
        printf("File could not be opened\n");
        return 1;
    }

    // Read header
   WAVHEADER in_wav_header;

   fread(&in_wav_header, sizeof(WAVHEADER), 1, inptr);

    // check if input file is WAV file
   if (check_format(in_wav_header) != 0)
   {
        fclose(inptr);
        printf("unsupported file format\n");
        return 1;
   }

    // Open output file for writing
    FILE *outptr;
    outptr = fopen(output_name, "w");

    if (outptr == NULL)
    {
        printf("error creating output file\n");
        return 1;
    }

    // Write header to file
    fwrite(&in_wav_header, sizeof(WAVHEADER), 1, inptr);

    // Use get_block_size to calculate size of block
    int block_size = get_block_size(in_wav_header);
    printf("%i\n", block_size);

    // declare buffer array
    BYTE *buffer = malloc(sizeof(block_size));

    // set cursor to start of last block of input audio file
    fseek(inptr, -block_size, SEEK_END);

    // iterate through input file blocks, checking for header, and writing blocks
    // to the output file

    while (ftell(inptr) >= sizeof(WAVHEADER))
    {
        fread(buffer, block_size, 1, inptr);
        fwrite(buffer, block_size, 1, outptr);
        if (fseek(inptr, 2 * -block_size, SEEK_CUR))
        {
            return 1;
        }
    }

    // close files & free malloc space
    free(buffer);
    fclose(inptr);
    fclose(outptr);

}



int check_format(WAVHEADER header)
{
    char check[] = {'W', 'A', 'V', 'E'};

    for (int i = 0; i < 4; i++)
    {
        if (header.format[i] != check[i])
        {
            return 1;
        }
    }
    return 0;
}

int get_block_size(WAVHEADER header)
{
    if (header.numChannels == 1 || header.numChannels == 2)
    {
        int block_size = (header.bitsPerSample / 8) * header.numChannels;

        return block_size;
    }
    else
    {
        printf("error: too many audio channels\n");
        return 1;
    }
}