You do need to ensure that mic setting is as Ray describes. However, the third sentence in Ray's reply may well be the critical one for you: The the Windows sound settings make sure Stereo Mix (or your systems equivalent) is selected as the input device. Sometimes it is named "What U Hear".
For some computers this setting is very well hidden if it is available at all! (Microsoft were "strongly persuaded" by a number of record companies to remove this facility since they asserted it allowed users to make bootleg copies of copyright material, even if the users had actually bought the original CD.)
IOW, this is NOT a problem with your Magix software, rather that "someone" has decided that you should not be able to "easily" record the audio (or video) playing on your computer.
As Jeff has pointed out, the 'Stereo Mix' or 'What You Hear' setting is normally invisible within the Windows sound control panel. You need to right click within any panel showing and select 'Show Disabled Device' for them to show and be enabled. That setting will enable any input to have access to the inboard sound mixer. Unfortunately without enabling apps access to the mic input that also disables the ability for the Stereo Mix to function.
It would not matter what screen recording program you used, it would not function without those options in use.
Actually...he says slightly sheepishly...I think it was me who disabled all of the inputs that I wasn't using, many moons ago. I re-enabled it from the sounds control panel and voila, it became an audio option and my recording is sorted.