@htt Yeah, but like I said, the "immediately/as soon as" meaning of this makes no sense.
First thing first, the sentence structure is {V1 (dic form) なり, V2} with V2 is the immediate consequence of V1. But there is no V2 here: "using earphones" is not the consequence, and even if you argue that the order of the actions doesn't matter, the link you provided says otherwise.
山頂に着く/いた ×なり 向かいの山の頭から太陽が昇ってきた
Unless the V2 is intentionally omitted, then the meaning would be really vague and I think the possibility is low.
Secondly, the link also says that なり is not followed by volition forms.
There is also a mentioning of なり = とか in the usage, and I think it is the most fitting.
Edit: thanks for the link anyway, I learned a lot from that.