In "Kupiliśmy" there seems to be a missing syllable: it sounds to me like "Kupiśmy". Is that regular Polish pronunciation?
I hear it. Plus, you don't really have vanishing syllables in Polish, even single sounds 'vanish' extremely rarely ;)
I have a similar issue (female voice, slow): the audio is distorted (overloaded?) around the ł of "Kupiłiśmy", making it practically impossible to hear the actual sound.
Errm. Is it not "Kupiliśmy" rather than "Kupiłiśmy"? Moreover, on this forum page, the sentence now seems to have been re-recorded using the somewhat better male voice.
The male/female voice is an A/B test, some users have the male one (like you and I), some still have the female one (like RobinB does).
The word srednia is translated to different words in English. It can be midsized, medium, middle, etc. Dependind on how it is used
"middle" (there are three sandwiches here, I would like the one lying in the middle) is "środkowa".
"medium" is the main answer, "mid-sized" and some other work.