The other day I had my monthly pizza, wine and music playing session with Hank Meldrum again.
[Established time: a day, with music playing]
I took this LP with me which I hadn't played in say 10 years.
[Established time: not changed. We are still in the music playing session. This described something happening in the session.]
It didn't hit me then.
[We were describing the session, which we know to be in the past from the phrase "the other day." So, "then" refers to during the session.]
You want to know if the word "then" means the other day when he had his monthly pizza, or before when he first listened to the record about ten years ago. You correctly intuit that "then" refers to when he played the record on the day when he had his monthly pizza. FumbleFingers correctly points out that it is ambiguous, but there are a couple hints.
If the author meant 10 years ago, they likely would have used some other form, such as:
- I took this LP with me which I hadn't played in say 10 years; it didn't hit me then. [Still ambiguous]
- I took this LP with me which I hadn't played in say 10 years. It didn't hit me back then.
- I took this LP with me which I hadn't played in say 10 years. It hadn't hit then.
More context / more sentences before and after would help to understand for certain what the author meant.