I usually use Cheese for screen-casting audio/video
For straightforward audio editing, I usually use Audacity
For video editing I usually use Openshot or Kdenlive
For video conversion, I usually use Transmageddon
For midi editing and playback I use Muse and Rosegarden
For midi keyboard stuff, I use a combination of QjackCTL, Qsynth, VMPK, Rosegarden and my external midi keyboard with a usb connection. This setup is worth a video in in itself to explain properly. It works beautifully in terms of meeting my needs. I should say, i also use Zynaddsubfx synth with my keyboard