Recently I found this application, OBS Studio, for screencasting. It is the best screencasting software I found, and it is cross-platform.
In Linux, I tried several applications before, recordMyDesktop, SimpleScreenRecorder, and even ffmpeg command-line, and none of them is satisfying. The main issue is either audio recording not synchronised or audio difficult to configure.
OBS Studio is user friendly and easy to configure and the audio is synchronised with the video. Great!