„Z” generally has two meanings:
„z” + genitive – 'from' 'made of'; „Piwo jest z browaru” = 'Beer is from the brewery'; „Człowiek ze stali” = „Man of Steel”; „To jest z drewna” = 'It's made of wood'.
„z” + instrumental – 'with' „Kanapka z mięsem” = (lit.)'A sandwich with meat'; „Kobieta z brodą” = „A woman with a beard”; „Pies z kotem” = 'A dog with a cat'.
Instrumental on it's own, denotes the instrument you are using to perform action:
- „Kroisz chleb nożem” = 'You are cutting bread with a knife'; „Rzuca mięsem” = 'He/she/it is throwing meat'(also, an idiom = 'He/she/it swears heavily'); „Rozbił szybę młotkiem” = 'He broke a glass with a hammer'.
„Kanapka mięsem” would only work in something like „Kanapka mięsem go pobił” = 'Some dude nicknamed 'Kanapka' beat him up using meat' ie. a bit nonsensical.
I don't know how to confirm what "slow voice" tell.
But in Polish we don't pronounce z as "zed". "Zet" is a name of letter "Z". And is only spoken as zet when is used as an abbreviation, e.g. PZU = pe-zet-u; NFZ = en-ef-zet (but ZUS = zus), Jan Z. = Jan Zet.
I noticed that algorithm on Tinycards on Duolingo ALWAYS read "z" and "w" at the end of the sentense (as Z. = zet and W. = wu). And this is bug, because we don't say "Wracam z" as wracam zet or "pukam w" as pukam wu". Yes there are longer form z = ze and w = we but they are used when there is to many consonants, e.g.
wracam ze szkoły - I'm coming back from school.
wracam z wycieczki - I'm coming back from a trip.
wracam z zabawy - I'm coming from a party.
wracam ze Zbyszkiem - I'm coming back with Zbyszek.
pukam we drzwi - I knock on the door.
pukam w okno - I knock on the window.
mieszkam we Włoszech - I live in Italy.
mieszkam w Warszawie - I live in Warsaw.
PS. Personally I don't know anyone who use names of Polish letters when spelling (maybe in elementary school). People use names :), e.g.
K jak Kasia
W jak Wiesiek
I jak Irena
A jak Antek
T jak Tomasz
K jak Konrad
O jak Olek
W jak Wacek
S jak Stanisław
K jak Kamil
A jak Ania
PS.2. Before I went to elementary school and I was taught proper names I would say (and I'm still saying :) ): by, cy, py, zy etc. You must know that we use Polish letters for ~500 years and spoken Polish is at least two times older :). So the names of letters aren't something "natural" :)