Green recorder uses the default audio device you have to record. So if you want to change the audio input source, you just need to change it from the system-side (using pavucontrol for example).
By default, On Wayland only, Green Recorder uses the V8 encoder instead of the default V9 encoder in GNOME Shell because of the CPU & RAM consumption issue with V9. Which - now - should also give you better performance. On Xorg, each format uses its own default encoder.
This is a recording sample for DOTA 2, made by the developer of a Green Recorder.
Installation latest version into Linux Mint 18 and higher:
May the Force be with you,
sudo add-apt-repository ppa:mhsabbagh/greenproject sudo apt update; sudo apt install -y green-recorder