I had the same issue, and I will explain how I made it work in my system step by step. I hope this can help someone else.
I will leave the links to some websites that were very useful.
The global idea is to install JACK to manage the audio, and bridge JACK to Pulseaudio to be able to control the audio volume.
Step1. Remove Pulseaudio from the system to get a clean JACK installation:
sudo apt-get remove pulseaudio
sudo apt-get remove --auto-remove pulseaudio
sudo apt-get purge pulseaudio
Step 2. Install JACK:
sudo apt-get install qjackctl jackd2
qjackctl/stable,stable 0.9.1-1 amd64, the user interface for controlling the JACK sound server
jackd2/stable,stable 1.9.17~dfsg-1 amd64, JACK Audio Connection Kit (server and example clients)
During JACK installation, a windows open asking to Enable realtime process priority. I answered yes.
Step3. Once installed, execute JACK, and a graphical interface will open
Step4. Re-install Pulseaudio and module-jack
sudo apt-get install pulseaudio-module-jack
sudo apt-get install pulseaudio
Step5. Install Cadence software to allow bridging Pulseaudio/ALSA with JACK
sudo apt-get install apt-transport-https gpgv
sudo dpkg -i kxstudio-repos_10.0.3_all.deb
sudo apt install cadence
Step6. Configure the system. Here is where you will need to play around depending on your system audio devices.
On JACK, open settings (Setup...), and define your sound card. These are mine:
Bridge ALSA/Pulseaudio with JACK
Restart the system
Step 7. Configure Simplescreenrecoder to listen to your device. Check that it is getting audio from the system and microphone
Probably this will not work at the first time. Play around with the JACK and Cadence configuration.
Some links that helped me:
If you need something different, or I missed a step, please comment. This is not easy, and having clear steps would help so many people.
Good luck everyone