I think batteries are connected in series in a Walkman but I am not 100% possitive.

Also, how can I find out the total voltage of two AA batteries that are each 1.5V? Is there a formula?

If batteries are side-by-side, the voltage stays the same (e.g., 1.5v), but if they're stacked end-to-end, the voltage adds up (e.g., 1.5 + 1.5=3v, etc.). Batteries are normally "stacked" into a Walkman, at least in my experience. :)