为什么要设置通道个数？

告诉系统输出设备的使用通道个数，假如扬声器是4声道，设置通道个数为2，则会有2个声道没有声音。

立体声还是单声道？

源码分析

H:\webrtc-20210315\webrtc-20210315\webrtc\webrtc-checkout\src\media\engine\adm_helpers.cc
void Init(AudioDeviceModule* adm) 方法

webrtc::adm_helpers::Init

void Init(AudioDeviceModule* adm) {
**
    // Set number of channels
    bool available = false;
    if (adm->StereoPlayoutIsAvailable(&available) != 0) {
      RTC_LOG(LS_ERROR) << "Failed to query stereo playout.";
    }
    if (adm->SetStereoPlayout(available) != 0) {
      RTC_LOG(LS_ERROR) << "Failed to set stereo playout mode.";
    }
***
}
adm->StereoPlayoutIsAvailable(&available)
->
int32_t AudioDeviceModuleImpl::StereoRecordingIsAvailable(bool* available) const {
**
  bool isAvailable = false;
  if (audio_device_->StereoRecordingIsAvailable(isAvailable) == -1) {
    return -1;
  }
  *available = isAvailable;
***
==
}
->
int32_t AudioDeviceWindowsCore::StereoRecordingIsAvailable(bool& available) {
  available = true;
  return 0;
}

adm->SetStereoPlayout(available)

AudioDeviceModuleImpl::SetStereoPlayout

int32_t AudioDeviceModuleImpl::SetStereoPlayout(bool enable) {
  RTC_LOG(INFO) << __FUNCTION__ << "(" << enable << ")";
  CHECKinitialized_();
    // 上节课的 AudioDeviceWindowsCore::InitSpeakerLocked成功后，该函数就会返回true
    // 主要返回的是_speakerIsInitialized变量值
  if (audio_device_->PlayoutIsInitialized()) {
    RTC_LOG(LERROR)
        << "unable to set stereo mode while playing side is initialized";
    return -1;
  }
  if (audio_device_->SetStereoPlayout(enable)) {
    RTC_LOG(WARNING) << "stereo playout is not supported";
    return -1;
  }
  int8_t nChannels(1);
  if (enable) {
    nChannels = 2;
  }
    // buffer会根据通道的数量来计算大小
  audio_device_buffer_.SetPlayoutChannels(nChannels);
  return 0;
}

audiodevice->SetStereoPlayout(enable)，enable正常是true

int32_t AudioDeviceWindowsCore::SetStereoPlayout(bool enable) {
  MutexLock lock(&mutex_);
  if (enable) {
    _playChannelsPrioList[0] = 2;  // try stereo first
    _playChannelsPrioList[1] = 1;
    _playChannels = 2;
  } else {
    _playChannelsPrioList[0] = 1;  // try mono first
    _playChannelsPrioList[1] = 2;
    _playChannels = 1;
  }
  return 0;
}