[gentoo-user] Screenrecording with audio from firefox

Hi,

To record my screen with audio I installed 
obs and then simplescreenrecoder.

Recording the video stream more or less works (I cannot record a
firefox-window and need to record the full screen).

But recording the audio from the source which will be replayed
by firefox only give me a stuttering sinus tone.

According to different tutorials I watched the recording of
a gameplay works with video/audio which exactly mine setup.

Interestingly I cannot find the regarding options of both
screen recorders any more (the tutorials refer to older versions
of the recorders).

How can I record video and audio from the played source nonetheless?

Cheers!
Meino