I have not heard of any source in the suttas that sotapanna phala can only happen while listening, and Lal has not found it either.
I am of the personal opinion that it can also very well happen in meditation when reflecting upon the Dhamma one has heard before.
I think the reason why listening works so well though, is because you are actively sustaining in savittakka, savicara and get uplifted by the energy of the voice, and thereby approach the 1st Jhana. When reading the subtitles you are doing the same. Hence you feel the “Ariya effect” which is nothing but the arising of jhana factors due to savitakka savicara.
However when reading a post you do not get that sustained attention in the same way, since you don’t have the assistance of a voice to pay attention to. But I am sure many of you can relate to experiencing the same piti sukha when stopping and reflecting on what you just read. Hence I believe listening is not required for sotapanna phala to happen, but certainly immensely helpful.