Is it allowed on the leaderboards, or will it be rejected by the mods? There are some other games that require video and audio for verifications, I wonder if it's the same for SSB64?

I'd say it depends on the run. If it's an old run or an accidental cable unplug happened for one run, it could be accepted. It would be preferred to have audio / video for every run, but I think we could accept one run depending on how known the runner is. Video will always be the more important!

problem could be splicing audio in 1p runs

Splicing in 1P runs requires exact bonus points to carry over in between stages making it more difficult than most games

It's technically possible but effectively more difficult than just getting good enough to WR in most cases if not all

With my current setup I can only record game audio if I'm not doing commentary, for some reason this laptop when trying to record background noise to record my commentary and the sounds of my controller inputs, the game either is too loud for the other sounds to be heard, or the background noise is too loud and the game has inconsistent audio, it would be annoying to get a run rejected because of choppy audio issues so I guess when not sure just record the game audio and skip commentary unless you can get both to sound at the right volume levels without one overlapping the other right?

You would need obs and a mic to get both the game audio and you talking.

Otherwise if unsure, just go with the game audio instead.

Well my runs were approved anyway despite the too low or too high audio that would vary between levels, I will consider improve my headset with something with a better mic so it records audio consistently instead of stuttering (see Puff VH1 or Link VH1 to see what I'm talking about)

I reviewed them, it's fine overall. It could be better obviously but it's still acceptable.