Hi!
I've just seen it, sorry if you had to wait for an answer. Your spelling is pretty good, bravo

You should not bother yourself that much with the accent or short/long vowels. They vary from place to place, there's rich landscape of regional accents and dialects.
You'll simply pick them up with time, without thinking about it at all.

In the example words in your recording - grad, luk... are homonyms. They are deliberately used as examples to stress that if you say short U in LUK, it means onion. LUK with long U means arc. GRAD with short A is town/city, GRAD with long A is hail.

But there are not many homonyms in Ser/Cro, so even if you spell it wrongly, everyone will understand what you want to say.