You may need to figure out video formats, as I think you will find the common/easiest format to be MP4, which MX7 doesn't support. (MX8 does) But you may need to search for a downloader app that offers a useable format, or use a converter.
Audio is easy, most of them support grabbing the MP3.
You need a screen capture program, I use Litecam HD that records video and audio to separate tracks so downloading both game tracks then extracting the audio as an individual file for a mix and edit should do the trick.
There's quite a few programs all with a free trial, just google screen capture.
This was recorded with Litecam HD and I deleted the video first to leave only audio then saved as a flac, then imported back to Mixcraft and finally added the complete vid/audio file for comparison. The grey track is the original and the orange is the flac that obviously could be any extracted audio from any vid.
Mark wrote: When I import a video into Mixcraft it automatically splits off an audio track.
I had no idea about the you tube downloader and I realised sometime later that the audio would be split anyway meaning its not a special feature of a screen capture program so that's an education for me for wich I thank you kindly.