Kot- singular nominative ( subject of the sentence or in the "to" sentences)
Koty - plural nominative=accusative ( accusative- direct object for most verbs , and after some prepositions)
Kotem - singular instrumental, among other things, you use Instrumental after "jestem/jesteś/jest"
There is no form like "jestesz". Yes, a lot of people have troubles not only differentiating between ś and sz, but even perceiving the difference. But from a point of view of a Polish person that doesn't have any contact with foreigners trying to speak Polish, the sounds are totally, obviously different...
Try here, section 2 has comparisons of sounds. If you listen to the whole alphabet, disregard the weird 'ch' (my opinion is that it's weird), it's the same sound as normal 'h'.
The audio on this one is misleading. If the tone of the voice at the end is raised the sentence would be ('Are you a cat?') However if the tone isn't raised it translates to ('You are a cat'). On this audio, the tone isn't raised at the end so it should translate to (You are a cat). My polish girlfriend made me aware of this