AirPods Pro and speech central

Hello everyone, I have an iPhone 14. I use the Speech Central app for reading books, along with the eSpeak NG voices (Max) set to the Croatian language. The problem arises when I connect my phone to AirPods Pro headphones. At that point, the voice of the screen reader (eSpeak NG) sounds normal until I open a book in the Speech Central app and press play. Then, the voice I hear sounds as if it's coming from a well. There's some echo, and it's much harder to understand what it's saying. I'm wondering if anyone else has a similar issue and if there's a way to solve it. Let me emphasize that when I'm not connected to headphones, everything sounds normal.
I also have another question about the Speech Central app: How can I reduce the pause that eSpeak NG makes between sentences while reading a book? After every sentence, there's a 1-second pause before it continues to the next sentence.