Don't worry, went through the same pain, and Microsoft Support people are not smart enough to do get a proper IT job and have worse skills than your great grand mother who has never even seen an electronic device.
9/10 times its cause Teams is coded so badly that it tries to go to headset rather then stay on your default sound option
You can disable this in your System-Sound-Devices
Alternatively disable it
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Bluetooth headphones often have two modes: stereo (high-quality audio) and hands-free (low-quality audio with mic). Disabling the hands-free mode can prevent the system from switching modes.
- Open Control Panel and go to "Devices and Printers."
- Right-click your Bluetooth headphones and select "Properties."
- Go to the "Services" tab and uncheck "Handsfree Telephony."
- Click "Apply" and "OK."
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If you struggling to find it the above (Due to Microsoft making it harder and harder to get to the actually settings)
Control Panel -> Devices and Printers -> Devices - > More devices and printer settings (Scroll down) -> Continue from point 2 above