Apple - FaceTime calls lower system volume

Open FaceTime app (don't make the call yet), Paste the following line in to terminal,

printf "p *(char*)(void(*)())AudioDeviceDuck=0xc3\nq" | lldb -n FaceTime

and then make the call.

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Update: for macOS sierra or higher, replace FaceTime with avconferenced.

On current macOS (Mojave and Catalina), System Integrity Protection doesn’t allow modifying the running FaceTime app. You will need to route the audio to another app to remove the ducking or amplify it.

One tool is called Loopback app from Rogue Amoeba. In FaceTime app, pick Loopback virtual device as the output so you can get control of the volume.

I am not affiliated with the developer, just a happy user who can't believe how useful this audio app is.