We are allowed to use music or sounds made by others but we need to clarify that the music/sound we used isn't made by ourselves right?

Only that we will get poor ratings on the sound/music category am I right?

Yes you can use already made assets according to the blackthornprod video.  He does ask that you give credit where credit is due.

but do make sure you have the right to use it, might run into trouble if you use licenced music or sound effects.