Regarding adding music to video that already has dialogue and sfx, is there a good, free or inexpensive video editor that can do this job for different frame rates and export formats (e.g. for HD, for YouTube etc) ?
There's plenty of free tutorials online and if you learn a proper video editor you'll be in a much better place if you want to pursue a different career.This one is complex, but therefore also full of professional options.
For item 1. Check the project settings of Resolve before you import your files, particularly the frame rate. You need to have the same settings in both Cubase & Resolve.
2. I can only think of obvious things like making sure the audio isn't muted in the timeline. Try with different simple source material - import a video with sound and then render it - same result?
3. That's annoying and wierd. Check your sound settings and try changing to the native driver for your machine and listen with headphones etc.
I'm no expert, but that's what I would try.