[WASAPI]If set recording device to built-in speaker, a flat waveform will be recorded before playback

To record output of pc(youtube), I using WASAPI.

But I have a problem. If set recording device to an external speaker(KENWOOD C-535 connect with bluetooth) and click recording button, recording will start when playback start(below image ① and ②). In other words, recording will not start until playback start.
but if set recording device to built-in speaker(realtek(R) audio), a flat waveform will be recorded when click recording button in spite of before playback(below image ③).
↑image ① : recording device is C-535 and record don’t start until playback
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↑image ② : recording device is C-535 and record start when playback start
↑image ③ : recording device is realtek(R) audio and record flat wave in spite of playback don’t start

Environment : Audacity 2.4.2, Windows10 x64

windows10 audio volume setting menu show below. I setting mute else of chrome(for youtube) and audacity by toggle each speaker icon(red circle).
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I tried to change Sound Activated Recording setting with recording device set to [realtek(R) audio]

If set Sound Activated Recording to -59dB, recording didn’t start until playback. But, if set Sound Activated Recording to -60dB, recording start in spite of before playback(flat wave recorded). And In my 1st post of this topic, Sound Activated Recording was set off(didn’t put a check mark).
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↑Sound Activated Recording(Preference menu)
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↑Sound Activated Recording setted to -59dB didn’t start recording until playback.
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↑Sound Activated Recording setted to -60dB start recording in spite of didn’t start to playback and recorded flat wave

WASAPI is different from the other Windows sound hosts in that it is not always active.

If you start Audacity recording from WASAPI, Audacity will capture samples from the selected device. If the selected device is not producing a stream of samples, Audacity will wait for samples to arrive.

With your realtek device, “something” is using the device - it may just be monitoring one of its inputs (In the Windows Sound control panel, the realtek may have “listen to this device” enabled) - it may be that your web browser is accessing the device - it may be something else.

Thanks for replying. I checked control panel and in mic(amd audio device) of record tab, there is no checked “listen to this device” checkbox. And There isn’t “listen to this device” checkbox in speaker(realtek(R) audio). I will investigate a little more to figure out what software using realtek(R) audio device.