[bandito2]

Sounding more like a mostly broken off gear tooth. The clack, clack, clack is from gear teeth slipping or skipping and banging up against the next good tooth. At slow speed there is more time for good teeth to slip/skip past the empty space. That squeaking noise sounds like gear teeth ends grinding against the broken edge and pushing at the tooth either side of the gap (depending on acceleration or deceleration) since there is not full meshing contact. The chipped/broken tooth is being eroded.... That is my interpretation anyway.

Think Car starter problem of teeth missing from the flywheel. At some point more teeth are going to break off. And it would be possible if positioned just right, the bike would not move because there is no gear meshing to drive the rear axle...... until the bike gets rolled and the gear turns enough for teeth to start meshing again... Of course with the skip and clang effect even worse.

If that is not what is happening and I have misinterpreted the description and videos, then it could be the (very bad) bearings after all.

Here are 2 videos where bearings and some performance gears are replaced on a KYMCO Super9. Access to the gears may be different than this for your bike, but again, the steps are going to be pretty much the same.

1) remove covers to gain access to clutch
2) remove drive belt and clutch
3) remove rear wheel
4) drain transmission
5) remove final drive transmission cover to access gears
6) pull gears to access bearings
7) pull bearings needing replacement
8) clean case of foreign material before replacing bearings
9) clean gears before replacing
10) after bearings and gears are installed, the rest is essentially reverse order of steps 5 to 1.

Some of this work may require some specialized tools especially if replacement gears are needed. However, stock shafts with the the necessary gears already installed might be available. Not sure with this bike. A service manual would really be helpful to do the examination parts of this at least so that you'll not be clueless if you take it to a shop for repair. It will be essential for proper procedures and torque values should you decide to do the work yourself.
HTH