I looked at the rules and one of them is to keep the original game audio. But recently I've seen more and more verified speedruns with music added. Is that rule no longer true?

I guess

You have to keep original game volume, so i think it should just be a max volume

You have to have the game audio in it even with it without music. I think they let the squad any% WR for GT slide cause of it being really close ig but I’m not 100% sure.

You are allowed to add custom music to speedruns, as long at the ORIGINAL quest music can be heard (loud or quiet)

Basically adding music to it or listening to music as you are running is okay. The rule is referring to completely removing or replacing audio. To address runs that break this rule, there is usually a video with game audio given to the verifiers (The main reason for this being a microphone unintentionally being on).