The word you hear is "nix," which is pronounced "nicks" or "nics". This is colloquial German, not formal.
"Nichts" is pronounced with the whistling "ch" (tough tip of tongue to lower teeth and arch towards the pallet, then exhale) and the "ts" sound that is the same as the German "z" (start with the tongue in the same position you would start a "t", but then pronounce an "s", making your tongue slide down your upper teeth and release an explosive before the "s" sound).
https://www.youtube.com/watch?v=nAC2-LgyO-k