In the tips & notes, word stress was explained. I remembered that the /'lai/ syllable would be stressed in these words. But when I read the sentence, I guessed the secondary stress would be on the /lan/. (is-lan-ti-lai-nen). So I was surprised to hear it on the /'is/ instead (is-lan-ti-lai-nen). However, that makes sense, if you treat the islanti part of the word on its own... three syllables, stress the first one. Right?
Yes, the most important stress is always on the first syllable. I wouldn't say there really is any stress on other syllables. It may sound a bit like that, but there really is no stress, not even on "lai". Finnish is a bit "angular" language with pretty hard spoken consonants, so it may sound like stressing, but it is not.