Run the check
Record with speakers muted while desktop audio plays.
Repeat with headphones and the same input selected.
Inspect any system loopback or stereo-mix input and disable one route at a time.
Read the result
Sound surviving muted speakers suggests a software capture path rather than acoustic leakage.
Limits and next step
The browser cannot inspect hidden virtual-cable or call-app routing.
Repeat the check after changing one setting or connection. Keep a record of what changed before deciding whether the device needs service.