When trying to use the SinkWriter for multiple audio streams:

            for (int i = 0; i < alternatives; i++)
            {
                DWORD ni = 0;
                CComPtr<IMFMediaType> co;
                MFCreateMediaType(&co);
                pMediaTypeOutAudio->CopyAllItems(co);
                hr = pSinkWriter->AddStream(co, &ni);
                if (FAILED(hr)) return false;
                AlternativeOutAudioStreamIndex.push_back(ni);
            }

Success.

Later on:

            hr = pSinkWriter->WriteSample(wheretowrite, pAudioSample);

where wheretowrite is the index of the output stream, in a loop to write all audio. Calls succeed.

The resulting mp4 file’s media info shows this for the second audio stream: Errors : Missing ID_END.

Where, for the first audio stream, it shows correctly mdhd_Duration : 29931.

What am I missing? The SinkWriter’s Finalize() method also succeeds. No error logs.