I cannot hear any difference on थि and थी
The audio sounds exactly the same to me. Is it just me? What is the difference?
I suppose one of them should have a slightly longer "i"-sound in the end, but i just can't hear it in the audio that is being played.
In sanskrit, the long and short i is more prominent in term of "matra" or time spacing with respect to "swara" or accent. I agree with you though, both diacritical marks sound the same. In conversational Hindi, there is generally not much difference between the two vowel sounds, especially if someone is speaking really fast. But I wish they included the sanskrit-oriented differences in length for beginners.
Well they are very similar sounds, so even if the audio is in fact different, it might be hard to notice. That's the only vowel that sounds super similar between short and long versions though in my opinion, other ones are very distinct, so I would say your brain will just figure it out based on context.