Duolingo sound is quite bad (with pauses/delays)

[Mopf]

Hi,

I have OSX 10.9.5 and using now Vivaldi. I like it a lot, but there are some sound issues with the language learning page duolingo.com. The page is reading the sentences, to get to know how to pronounce everything. Also there are listening-tasks, where you have to hear at the sentence, and write what they are saying.
The problem is, that the sound has some gaps/delays. It is not one complete sentence, in 50% of the cases, the sentence has some gaps, which is really annoying, because if a gap is in the mid of a word, or if you have more than one gap in a sentence, it is hard to understand.

I don't know what is the problem. In all my other browsers it works well.

Maybe somebody has an idea. Thank you.

Greetings
Mopf