Translation:I looked out the window, and I saw my cat.
I think this could just as reasonably be translated as "I was looking out the window(didnt just do it but have been sitting there for a while) and I saw my cat. Je regardais..... Is the audio so distinguishably different to make me wrong? I can't tell by the audio.. and believe grammatically the imperfect also fits.
You're right that the imperfect would also be grammatically fine here, but there are a couple of differences in pronunciation between "j'ai regardé" and "je regardais". So based purely on the audio, it would be "j'ai regardé" and not "je regardais".
The more noticeable pronunciation difference is in the ending vowel sound of "regardais" vs. "regardé". The "-ais" of "regardais" is pronounced like "è" (with an accent grave), similar to the "e" in the English word "set". The "-é" of "regardé" (with an accent aigu) is pronounced similar to the "ay" in the English word "say". There's also a difference in the vowel sound between "je" and "j'ai", though at faster speaking speeds, it's not as easy to distinguish. The vowel sound of "je" is a schwa, similar to the "a" in the English word "about". The vowel sound of "j'ai" is pronounced like "é", again similar to the "ay" in the English word "say".
It's maybe easiest to hear the difference between the two when entered into Google Translate, where you can click the Listen button to listen to the contrast between the two phrases.