I suggest you use SDL_Mixer for your audio. It will mix your sounds automatically etc.

http://www.libsdl.org/projects/SDL_mixer/

Here is a simple one that should work:

/*
* Sample of SDL_Audio. Compile and link with an SDL_main.
*
* Author: Pierre G. Richard
* Written: 4/12/2004
*/

#include <memory.h>
#include "SDL.h"

typedef struct {
Uint8 *buffer;
Uint8 *bufMax;
Uint8 *bufCurr;
SDL_sem * playEnded;
int isEnabled;
}CallbackData;

/*-------------------------------------------------------------------callback-+
| "Play" callback |
+----------------------------------------------------------------------------*/
static void callback(void * udata, Uint8 * stream, int len)
{
CallbackData d = *(CallbackData *)udata;
if (d.isEnabled) {
if (d.bufCurr < d.bufMax) {
if (d.bufCurr+len > d.bufMax) {
len = d.bufMax - d.bufCurr;
}
((CallbackData *)udata)->bufCurr += len;
SDL_MixAudio(stream, d.bufCurr, len, SDL_MIX_MAXVOLUME);
}else {
d.isEnabled = 0; /* disable self */
SDL_SemPost(d.playEnded);
}
}
}

/*-----------------------------------------------------------------------main-+
| |
+----------------------------------------------------------------------------*/
int main(int argc, char ** argv)
{
SDL_AudioSpec wave;
CallbackData d;
int bufSize;

if (argc != 2) {
fprintf(stderr, "Usage: playSound <soundFilePath>\n";
}else if (!SDL_LoadWAV(argv[1], &wave, &d.buffer, &bufSize)) {
fprintf(stderr, "Error %d loading the sound file.\n", SDL_GetError());
}else {
SDL_AudioSpec device;

device.freq = 22050; // Samples per sec (FM Radio quality)
device.format = AUDIO_S16; // Signed 16-bit little-endian samples
device.channels = 2; // Number of sound channels (stereo)
device.samples = 4096; // Size of the Audio buffer in samples
device.callback = callback; // "Play" callback
device.userdata = &d; // callback data

d.isEnabled = 0; // disable our callback
if (SDL_OpenAudio(&device, 0) < 0) {
fprintf(stderr, "Error %d opening the audio.\n", SDL_GetError());
}else {
d.playEnded = SDL_CreateSemaphore(0);
d.bufCurr = d.buffer;
d.bufMax = d.buffer + bufSize;
d.isEnabled = 1;
SDL_PauseAudio(0); // make sure we play
SDL_SemWait(d.playEnded); // wait here until the callback finishes
SDL_DestroySemaphore(d.playEnded);
SDL_CloseAudio();
}
SDL_FreeWAV(d.buffer);
}
return 0;
}

/*===========================================================================*/