I've mapped pin12 and pin13 to play audio! woot.

The following code is not mine and taken from:

https://www.raspberrypi.org/forums/viewtopic.php?f=44&t=39138

I just want to have everything in one place and have a copy of it somewhere.

/*
Utility to switch Raspberry-Pi GPIO pin functions
Tim Giles 01/04/2013

Usage:
$ gpio_alt -p PIN_NUMBER -f ALT_NUMBER

Based on RPi code from Dom and Gert, 15-Feb-2013, 
and Gnu getopt() example 
*/

#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>

#define BCM2708_PERI_BASE        0x20000000
#define GPIO_BASE                (BCM2708_PERI_BASE + 0x200000) /* GPIO controller */
#define PAGE_SIZE (4*1024)
#define BLOCK_SIZE (4*1024)

int  mem_fd;
void *gpio_map;
volatile unsigned *gpio;
void setup_io();

// GPIO setup macros. Always use INP_GPIO(x) before using OUT_GPIO(x) or SET_GPIO_ALT(x,y)
#define INP_GPIO(g) *(gpio+((g)/10)) &= ~(7<<(((g)%10)*3))
#define OUT_GPIO(g) *(gpio+((g)/10)) |=  (1<<(((g)%10)*3))
#define SET_GPIO_ALT(g,a) *(gpio+(((g)/10))) |= (((a)<=3?(a)+4:(a)==4?3:2)<<(((g)%10)*3))

#define GPIO_SET *(gpio+7)  // sets   bits which are 1 ignores bits which are 0
#define GPIO_CLR *(gpio+10) // clears bits which are 1 ignores bits which are 0



int main (int argc, char **argv) {
  int opt, flag, n_pin, n_alt;
  flag=0;

  while ((opt = getopt (argc, argv, "hp:f:")) != -1) {
    switch (opt) {
    case 'h':
      break;
    case 'p':
      n_pin = atoi(optarg); flag |= 0b0001; break;
    case 'f':
      n_alt = atoi(optarg); flag |= 0b0010; break;
    case '?':
      // getopt() prints error messages, so don't need to repeat them here
      return 1;
    default:
      abort ();
    }
  }
  
  if (flag != 0b0011) {
    fprintf (stderr, "Usage:\n$ gpio_alt -p PIN_NUM -f FUNC_NUM\n");
    return 1;
  }
  
  setup_io(); // Set up gpi pointer for direct register access
  INP_GPIO(n_pin);  // Always use INP_GPIO(x) before using SET_GPIO_ALT(x,y)
  SET_GPIO_ALT(n_pin, n_alt);
  
  printf("Set pin %i to alternative-function %i\n", n_pin, n_alt);
  
  return 0;
}



void setup_io() {
   /* open /dev/mem */
   if ((mem_fd = open("/dev/mem", O_RDWR|O_SYNC) ) < 0) {
      printf("can't open /dev/mem \n");
      exit(-1);
   }

   /* mmap GPIO */
   gpio_map = mmap(
      NULL,             //Any adddress in our space will do
      BLOCK_SIZE,       //Map length
      PROT_READ|PROT_WRITE,// Enable reading & writting to mapped memory
      MAP_SHARED,       //Shared with other processes
      mem_fd,           //File to map
      GPIO_BASE         //Offset to GPIO peripheral
   );

   close(mem_fd); //No need to keep mem_fd open after mmap

   if (gpio_map == MAP_FAILED) {
      printf("mmap error %d\n", (int)gpio_map);//errno also set!
      exit(-1);
   }

   // Always use volatile pointer!
   gpio = (volatile unsigned *)gpio_map;
}

One time setup:

gcc -o gpio_alt gpio_alt.c
sudo chown root:root gpio_alt
sudo chmod u+s gpio_alt
sudo mv gpio_alt /usr/local/bin/

Cron that:

gpio_alt -p 13 -f 0
gpio_alt -p 12 -f 0