Thanks! I checked the other voices and only for the Jacek and Jan voices do I clearly hear no vowel. If I type "Ony nie" (I know "ony" isn't a word, but I just wanted to check) for the Maya voice, it sounds very similar to how she pronounces "On nie", but maybe it's just a trick of the ears...
It's because Polish people do not say double consonants as a one smooth long sound like "nn" (at least some people don't).
Instead, you make a short pause between any double consonants.
So, the name Anna can often sound when pronounced a bit like "Anana" (an ... na) ...but I've noticed this is not always the case - I suppose it's regional/generational thing.
Myślę, że ten link pomoże: http://www.lingvika.pl/tell-say-speak-talk-uzywac/ :)