im too broke to afford a cap card

I don't see the game footage lmao, if you mean pointing a phone at your 3ds then yeah that is allowed i think

as long as the game footage is visible enough to make out what is happening on screen, its allowed

Do you need game audio in the recording?

i'd say no, but id reccomend having it anyway

Yeah, Ideally I would have audio it's just the way my 3ds is on a stand I can't plug an audio line in. I'll try to fix it but it's good to know that if I do a run without audio before then it won't be rejected

Does the audio work in the video using aux cable?

Depends how you're recording it - if you're doing it from your phone you might be able to plug the aux into your phone and use it like a microphone