Hi FearlessAle, thanks for the questions! There's one answer to all of them, really, which is that yes, you can use pieces of the official trailer, the game soundtrack, or the audio track from an official trailer, but only alongside lots of your own original work - enough to make your submitted content materially different. It's tricky to know exactly where the line is, but I personally wouldn't use the entire audio track from an official trailer without any changes, for instance.
From a legal perspective, this is basically a question about copyright law. You can read more in our copyright policy and terms of use, as well as 'fair dealing' and 'fair use' in UK and US copyright law.