In most parts of Wales, except for the North West there is no difference in sound between the vowels 'i' and 'u'.
The computer based text to speech used on the course follows this rule and thus you will hear no difference in the vowel sounds.
This difference in vowel sounds is important because the highest concentration of Welsh speakers is in the North West.
This video explains this:- https://www.youtube.com/watch?v=Gb8Bps3bG84&list=PLz6oFM0_IszwxmU7dMcGQClZ5zMwX2EdY&index=2
Well, it would be dy gi du and dy gi di. In some dialects there would be no difference in the pronunciation. Moving up towards north and north-west Wales there would be an increasing difference between du and di - watch the Welsh Plus pronunciation videos that we recommend to hear the distinction in sound made in the woman's strong north-west Wales accent.
However, if you are going to use the second pronoun di then you would probably say dy gi du di and dy gi di in any case. Otherwise, you would probably stick with dy gi and dy gi du.
In practice it is not a particular difficulty because of the context in which a real conversation takes place, and because people quite quickly adjust to another person's accent, dialect and register of formality, just as you do in your own language.