The audio says "katselet" instead of "katselette". Reported
Also came here to report this - sounds like the end of -tte has been cut off. Reported with flag also.
Why is "mikä" wrong in this sentence?
"Mitä" is partitive form of "mikä". "Katsoa" is a partitive verb, so you have to use partitive with it while forming a question too.
Note that sarjaa is also in partitive.