怎样得到声卡上的IAMCrossbar接口

下面的代码会弹出no,为什么不弹出ok?ICaptureGraphBuilder2 *pBuild;//视频采集过滤器图表
IBaseFilter *pACap;//Audio Capture Filter我已经通过下面两个函数把声卡的Filter加入到pGraph中了pACap=GetAudioInputFilter(aFriendlyName);
hr=pGraph->AddFilter(pACap,L"Audio Capture Filter");
if(pACap)
{
IAMCrossbar *pXBar1=NULL;
hr=pBuild->FindInterface(&LOOK_UPSTREAM_ONLY,NULL,pACap,
IID_IAMCrossbar,(void**)&pXBar1);
if(SUCCEEDED(hr))
MessageBox("ok");
else
MessageBox("no");
}
else
MessageBox("声卡误");