alsa APIs

snd_pcm_open
snd_pcm_hw_params_set_format
snd_pcm_prepare
snd_pcm_writei
snd_pcm_drain
snd_pcm_close
https://www.alsa-project.org/alsa-doc/alsa-lib/group___p_c_m.html

issues

If your device can't support mixer feature, or pcm interface can't open multi times.
You can try PulseAudio.
pulseaudio example

compile cmd

gcc -o play play.c `pkg-config --cflags --libs alsa`

source

#include <alsa/asoundlib.h>
#include <stdio.h>

#define PCM_DEVICE "default"

int main(int argc, char** argv) {
  unsigned int pcm, tmp, dir;
  int rate, channels, seconds;
  snd_pcm_t* pcm_handle;
  snd_pcm_hw_params_t* params;
  snd_pcm_uframes_t frames;
  char* buff;
  int buff_size, loops;

  if (argc < 4) {
    printf("Usage: %s <sample_rate> <channels> <seconds>\n", argv[0]);
    return -1;
  }

  rate = atoi(argv[1]);
  channels = atoi(argv[2]);
  seconds = atoi(argv[3]);

  /* Open the PCM device in playback mode */
  if (pcm =
          snd_pcm_open(&pcm_handle, PCM_DEVICE, SND_PCM_STREAM_PLAYBACK, 0) < 0)
    printf("ERROR: Can't open \"%s\" PCM device. %s\n", PCM_DEVICE,
           snd_strerror(pcm));

  /* Allocate parameters object and fill it with default values*/
  snd_pcm_hw_params_alloca(&params);

  snd_pcm_hw_params_any(pcm_handle, params);

  /* Set parameters */
  if (pcm = snd_pcm_hw_params_set_access(pcm_handle, params,
                                         SND_PCM_ACCESS_RW_INTERLEAVED) < 0)
    printf("ERROR: Can't set interleaved mode. %s\n", snd_strerror(pcm));

  if (pcm = snd_pcm_hw_params_set_format(pcm_handle, params,
                                         SND_PCM_FORMAT_S16_LE) < 0)
    printf("ERROR: Can't set format. %s\n", snd_strerror(pcm));

  if (pcm = snd_pcm_hw_params_set_channels(pcm_handle, params, channels) < 0)
    printf("ERROR: Can't set channels number. %s\n", snd_strerror(pcm));

  if (pcm = snd_pcm_hw_params_set_rate_near(pcm_handle, params, &rate, 0) < 0)
    printf("ERROR: Can't set rate. %s\n", snd_strerror(pcm));

  /* Write parameters */
  if (pcm = snd_pcm_hw_params(pcm_handle, params) < 0)
    printf("ERROR: Can't set harware parameters. %s\n", snd_strerror(pcm));

  /* Resume information */
  printf("PCM name: '%s'\n", snd_pcm_name(pcm_handle));

  printf("PCM state: %s\n", snd_pcm_state_name(snd_pcm_state(pcm_handle)));

  snd_pcm_hw_params_get_channels(params, &tmp);
  printf("channels: %i ", tmp);

  if (tmp == 1)
    printf("(mono)\n");
  else if (tmp == 2)
    printf("(stereo)\n");

  snd_pcm_hw_params_get_rate(params, &tmp, 0);
  printf("rate: %d bps\n", tmp);

  printf("seconds: %d\n", seconds);

  /* Allocate buffer to hold single period */
  snd_pcm_hw_params_get_period_size(params, &frames, 0);

  buff_size = frames * channels * 2 /* 2 -> sample size */;
  buff = (char*)malloc(buff_size);

  snd_pcm_hw_params_get_period_time(params, &tmp, NULL);

  for (loops = (seconds * 1000000) / tmp; loops > 0; loops--) {
    if (pcm = read(0, buff, buff_size) == 0) {
      printf("Early end of file.\n");
      return 0;
    }

    printf("write data\n");
    if (pcm = snd_pcm_writei(pcm_handle, buff, frames) == -EPIPE) {
      printf("XRUN.\n");
      snd_pcm_prepare(pcm_handle);
    } else if (pcm < 0) {
      printf("ERROR. Can't write to PCM device. %s\n", snd_strerror(pcm));
    }
  }

  snd_pcm_drain(pcm_handle);
  snd_pcm_close(pcm_handle);
  free(buff);

  return 0;
}