I do not know if the rules changed in medieval Latin, but in classical Latin, at any rate, if the penultimate (the syllable before the last) is long, it gets the stress; if short, the syllable before it gets the stress. A syllable is long if it has a long vowel or if it has a consonant cluster. According to my dictionary, the u in mortuis (ablative plural of mortuus, dead) is short, so the stress is on the first syllable. That also happens to be the way I have always heard it pronounced; but I looked it up to be sure.