The artificial voice used on Duolingo has some limitations and a few oddities from time to time, but that is probably a price worth paying for having audio available for all sentences in the course. The section in the general discussion forum does point to some excellent materials for learning and practising pronunciation.
(Edit to add...) If you go to www.ivona.com (the source of the 'Welsh, Gwyneth' voice used on Duolingo), type the phrase into the box, then choose 'Welsh, Geraint', and then press 'Play', you may sometimes find that it sounds better or clearer.
It could be your connection or your version of flash. To me at normal speed it sounds like it's trying to say both 'si' with and without the 'sh' sound at the same time. In slow speed it is 'si' without the 'sh' sound. So not perfect but not too far off being correct.
There is actually nothing at all we can do about individual audio tracks, we have no access to them at all in the editor and Duolingo refuses, no doubt because of staffing issues to edit individual tracks.