In addition to the basic speakers and microphone for your computer, you may be connected to more than one speaker and microphone when using a Bluetooth headset.
When a Bluetooth headset is connected to a computer, the speaker and microphone are usually connected to the Bluetooth side, but when using it after disconnecting, you need to check the settings.
In that case, the sound related settings are checked, and you have to go into the system settings for both Windows and Mac.
I bought a Bluetooth headset and connected it to my MacBook, but after disconnecting the connection, I had to check the sound input and output beforehand.
So I looked for it.
There was a freeware called SoundSource.
Download SoundSource After installation, the headphone icon is displayed on the menu bar as shown in the picture.
You can not only check whether the input is the built-in microphone, or the output is the built-in speaker, but you can also select it.
This tool seems to be useful when two or more inputs and outputs are used each.