Use a virtual audio cable or loopback device
Set the audio source to play into the virtual cable
Set your microphone input to the virtual cable output
Enable stereo mix if your system supports it
Use a mixer app to route system audio to the mic input
Adjust input levels to avoid clipping
Test the setup in your target app
Disable microphone noise suppression if it blocks playback
Use push-to-talk if you want manual control
Monitor for echo or feedback and lower speaker volume if needed