Those owners of RME sound cards (such as RME Fireface UCX) who want to make screencasts, often face the problem of recording system audio. For example, when one runs a DAW (digital audio workstation) and tries to record both video and sound of a playing project, he/she finds out that audio is not recorded, just video.
There is a universal software mixer for RME sound cards – TotalMix, which can be tuned up so that one can record audio too. I show how to do that in my video here.