why Mix_OpenAudio always failed (sdl programing on linux)

int main ( int argc, char** argv )
{
    if(SDL_Init(SDL_INIT_EVERYTHING)<0)
        return 0;

    TTF_Init();


    pScreen=SDL_SetVideoMode(800,600,32,SDL_DOUBLEBUF|SDL_SWSURFACE);

    if(Mix_OpenAudio( 44100, AUDIO_S16, 2, 4096)<0)
        return 0;