Welcome to the Magix user to user forums, so not a contact for any Magix staff. Hence, it is not our own demos and we cannot fix it!
The fact that playback of the demos stops for you, but apparently not for anyone else it would seem, strongly suggests there is something on your computer that is not interacting well with MM.
So, as a first step, please read this post and come back with the FULL details of your computer and exactly which version and variant (Basic, Plus or Premium) of MM you have; the exact version number can be found under Help>About. Also, which audio driver are you using?
Then tell us exactly which demo you have tried to play, specifying also the exact point at which playback stops.
Yes, we are throwing a lot of questions at you but we cannot see or hear your computer and to be able to help we really do need a lot more information.