Imagine you're talking about an infinitely long sequence of numbers: 8, 4, 2, 1, 1/2, 1/4.... You can subtract each member from a starting value. For example, subtracting each from 16 leaves zero: 16-8-4-2-1-.5-.25... = 0 You could then talk about the sum of the sequence. You might say, "The sum is similar." 8+4+2+1+1/2+1/4+1/8... = 16
Not really, you may want to play it again. The voice is definitely pronouncing the u in Summe properly (like oo in book), not at all like a short-o (which sounds similar to the o in not), same with the e at the end of Summe, which is correctly pronounced as a schwa, and not that ah sound er makes. Ähnlich is pronounced fine as well, with the äh making that ay sound, while endlich has a clear eh sound like in bed. Also, endlich has a clear t sound missing in ähnlich.
Admittedly, the Duolingo voice is unnatural but the pronunciation, at least with this sentence, is good, so I'm not sure how you're hearing die Sommer ist endlich (note to other learners: this sentence would be incorrect; should be der Sommer ist endlich da).
Edit: DenisCurvello's reply leads me to believe that you may be talking about the male audio, I don't know about that one.
The female voice is the best. But I think we have to learn listening every different pronunciation as possible or maybe we just understand this woman. I don't understand British English very well because the Duolingo had just a American female voice. We can understand everyone in our native language because we learn with a lot of differents pronunciations every day.