I have discussed this issue in the post, “Four Conditions for Attaining Sōtapanna Magga/Phala“.
The latest information that I have is at #3 of that post, which I quote below:
“3. September 22, 2017: Previously, I had stated that one could learn about Tilakkhana by reading these days. That is still true and one could become a Sōtapanna anugāmi by reading.
– However, recently I came upon a dēsanā by the Waharaka Thēro which stated that a Sōtapanna anugāmi attains the Sōtapanna stage only while listening to a dēsanā by an Ariya (Noble person, i.e., one with at least the Sōtapanna stage).
– Apparently, a Sōtadvāra citta vithi of an Ariya (during a dēsanā) has the necessary javana power to act as a trigger. I am trying to find a Tipitaka reference, and I would appreciate receiving it from anyone who has that information. I will edit this post to include that reference when I find it.
– July 15, 2019: I still have not seen a definitive Tipitaka reference regarding this issue. However, all suttā on the conditions for attaining Sōtapanna stage list saddhammassavanaṃ (saddhamma + savanaṃ or “listening to Dhamma”) as one condition, as in #1 above. Since written texts were not available at the time of the Buddha, this is not definitive as a condition.
– However, Waharaka Thēro has mentioned that listening to a recorded dēsanā should count, per his opinion.”
Here is another key point:
Listening is not required to attain the Sotapanna Anugāmi stage.
It is also clear that a Sotapanna Anugami is also freed from the apāyās. See, “Sōtapanna Anugāmi – No More Births in the Apāyās”
P.S. In other words, a Sotapanna Anugami is guaranteed to attain the Sotapanna stage.