It depends somewhat on the context.
gehen is always walking at normal speed and would be used if you do not want to stress the process of going or walking itself, but just indicate that you go somewhere: ich gehe einkaufen, ich gehe zur Arbeit etc.
laufen can also mean walking at normal speed, but you would use it to indicate that you go by feet instead of using a car or the bus or something, for example. Nimmst du das Auto? - Nein, ich laufe.
laufen can also mean running though, especially in a sports context.
er ist die hundert Meter in unter zehn Sekunden gelaufen
(rennen also means running, but it is more informal than laufen.
Du bist ganz außer Atem, bist du gerannt? (= You're short of breath, have you been running?))
Wouldn't the auxiliary verb for ‘laufen’ become ‘haben’ if you use the verb transitively (in ‘er ist die hundert Meter gelaufen’)?
I seem to remember transitive (or transitively used) verbs always use ‘haben’ to form the Perfekt, but I may be mistaken.
Audio for "Tueren" sure sounds like she's saying "Dueren" (sorry, I don't have umlauts on my keyboard)
So, gehen and laufen are somewhat interchangeable, it seems. "We go through 21 doors" is not the same as "we walk through 21 doors".
Yes, to some extent you are right. As a native speaker, I would use "gehen", when someone walks - as in this case. You can use "laufen", when you are walking but almost running, e.g. on a marathon or when you are in a hurry, but not want to attract too much attention in public.
So in general "gehen" describes a movement slower than "laufen", yet to some extend you can interchange them.