Yo oigo 'llena', pero tienes que ser lógico cuando escuchas porque 'tiena' no es palabra española, así que hay que pensar en otra palabra.
The "LL" sound, like the "Y" sound, are often spoken as a "J" or "ZH" depending on the speaker's accent and nation of origin. "CALL NOW!" = Llama Ya! and often heard on Spanish language radio and TV, often as "Yama ya" and often as "Jama ja" or "Zhama Zha."
This sounds like "El tiene el jardin con flores". Would this be a legit sentence too?
Sadly, we in this forum can't help you with Duo's audio, or your equipment.
He fills up the garden with flowers Was rejected. But is it correct to be rejected?
The English is perfectly correct, but the tense and the meaning are slightly different. 'He fills the garden.....' implies that he generally fills the garden with flowers, while 'He is filling.....' implies that he is doing so at the moment, or will do so in the very near future.