Short answer: convert every video you want to import to a lower resolution MP4 first, using something like Handbrake
Somewhat longer answer: do what I said in the short answer above, but also match the video framerate and audio sample rate (in Cubase project properties). The most likely reason that audio is not imported from a video file is that the project audio sample rate is set differently to that of the video file you are trying to import. You can use something like MediaInfo to find out what the format of the audio in the video file is.
Much longer answer: there are so many video formats out there that you would have no time to make music if you were to begin to learn about all of them. Only when you examine all the variables in digital video formats will you appreciate how lucky we audio people are with only a handful of generally compatible audio formats to have to deal with!
Here are a couple of things to consider:
- if you’re composing against a video track, you probably don’t need a full resolution (e.g. 4K) video file
- the easier it is to decode and display a video track, the less likely you are to have audio glitches
Now, that second item is a potential problem right now because the one format that appears to work consistently with the new video engine is MP4, which can put a significant hit on the CPU while decoding, so the only thing you can do to compensate is, for example, convert to a lower-resolution format. All you really need is to be able to see where you are in the video, and with this new engine you can’t replace the audio in the original video, so the video track really is only a guide.