tuhaf bir sesin beni uyandirmasina ne kadar şaşirdigimi tahmin edebilirsiniz
The only think I do not understand here is why uyandirmak has "-masina" as a suffix
My knowledge about such construction does not give sense to the sentence
uyandirma (short infinitive) +si (someone's) +na (dative) ( I guess since "ne kadar)
but, that will mean "someone's sleeping", while in the sentence it is the sleeping of the one who is telling the story, the "beni".

What am I missing?

Question

tuhaf bir sesin beni uyandirmasina ne kadar şaşirdigimi tahmin edebilirsiniz
The only think I do not understand here is why uyandirmak has "-masina" as a suffix
My knowledge about such construction does not give sense to the sentence
uyandirma (short infinitive)  +si  (someone's)  +na (dative) ( I guess since "ne kadar)
but, that will mean "someone's sleeping", while in the sentence it is the sleeping of the one who is telling the story, the "beni".

What am I missing?

Answer

You almost got it, except +na(dative) has a different meaning. Think of it as phrasal verbs, having a particular preposition. sasirmak (to be surprised) is used with dativ. Bir seye sasirmak

birinin beni uyandirmasi = someone waking me up

sasirmak = to be surprised

I am surprised to see you sleeping = senin uyuyor olmanA sasirdim
I am surprised that you went there = oraya gitmenE sasirdim