class audio
{
public:
     int play(char *filename);
     int test;

private:
     static void	*audioThread( void *ptr );
     void		playAudio(void);
     pthread_t 		playThread;
}

void *audio::audioThread( void *ptr )
{
     audio *play = (audio *)ptr;
     play->playAudio();
     delete play;
     return NULL;
}

void audio::playAudio(void)
{
     test = 0;
     /* playback code */
}

int audio::play(char *filename)
{
     ....
     ....
     test = 1;
     if (pthread_create( &playThread, NULL, &audio::audioThread, NULL) != 0)
     {
             return -1;
      }

     return 0;
}

void *audio::audioThread( void *ptr )
{
     // ptr is NULL here
     audio *play = (audio *)ptr;

     // now you call playAudio() through a NULL pointer 
     play->playAudio();
     delete play;
     return NULL;
}

void audio::playAudio(void)
{
     // here the 'this' pointer is NULL, hence you have no access to
     // member variables, so the seg-fault is coming up ...
     test = 0;
     /* playback code */
}

int audio::play(char *filename)
{
     test = 1;

     // Here it starts to go wrong, because you pass in 
     // NULL as argument to audio::audioThread() ...
     if (pthread_create( &playThread, NULL, &audio::audioThread, NULL) != 0)
     {
             return -1;
      }

     return 0;
}

if (pthread_create( &playThread, NULL, &audio::audioThread, this) != 0)