Most vocal samples you will find in sample packs are pretty average / full of “uh-huh”, yeah baby" etc.
Sampling, including vocals is an artform. Go find movies, acapellas, non-royalty clips, etc.
But if you want to have a look around, you could try here try here for starters?
The only Octatrack specific sample packs I am aware of are the six from Loopmasters; which cover different genres. (No comment on any of them as I don’t have any of them)
Not sure about the mk1, the mk2 definitely has balanced inputs. If you are using a 3.5mm TRS to 1/4" TRS lead then that will result in very low (and distorted) audio.
The OT thinks you’re giving it a balanced signal, which means it takes the difference between the Tip and Ring signals. If those are actually the Left and Right signals from a stereo source then most of the signal will be cancelled and all that’s left is the part of the signal that is panned left or right, i.e. not equally present on both channels.
Get a cable that splits a 3.5mm stereo into two 1/4” mono jacks. Then either plug in both jacks (to sample in stereo) or just one (to sample either the L or R channel on its own).
While I haven’t dug into them yet, apparently Iris 2 from Izotope has some killer stock vocal samples. They have a big bundle on sale for $49 USD for a couple more days. I picked it up but haven’t jumped in yet. Others here have praised the stock vocal samples from it.
You can work around this problem by panning your phone audio output all to one side. If you have an iPhone, go to Settings > Accessibility > Audio/Visual and set balance all to one side.
Or get a proper cable. I couldn’t get one because of the lockdown so I just use this workaround.