"Welcome and thank you, Professor!"
Translation:Fàilte agus tapadh leibh Ollaimh!
It's regional. There are a few people pronouncing sentences on this course—only one of the women pronounce it with the ‘y’ sound as far as I've heard. It's not actually related to gender. Each speaker enunciates the g to a different degree.
Listening to agus in the Words list, you can hear for example that the guy pronouncing ‘cat agus muc’ hits the ‘g’ to a much softer degree than the guy who pronounces ‘cat agus balach’, though not quite so softly as the woman who pronounces ‘muc agus caileag’ to such a light degree that it has a ‘y’ quality (which, if you think about whereabouts in the mouth ‘y’ and ‘g’ are articulated, is an easy variation to envision).
It is in the vocative, only there's no vocative particle—the vocative particle ‘a’ is not used when the following sound is a vowel. Sometimes the vocative form of a word just happens to appear the same as the nominative.
That said, it is different in this case. Note that the word in the sentence is ‘Ollaimh’ with an ‘i’, rather than simply ‘Ollamh’. Aside from the vocative particle, masculine nouns are also slenderised when in the vocative (i.e. nouns that end in a broad consonant, which is a consonant neighbouring the vowels ‘a’, ‘o’, or ‘u’, have an ‘i’ inserted before the consonant to slenderise it.
This is why we have both the names Seamas and Hamish (which is an Anglicised respelling of the vocative version, ‘Sheamais’) in English—the latter originates from the vocative form.
Nominative: ‘Tha Seamas beag.’ Vocative: ‘A Sheamais! Tha thu beag!’
I was just about to comment asking for clarification on the difference in pronunciation of "Ollamh" and "Ollaimh".
Personally I don't hear a difference from speakers, but my experience from other words would indicate that at least in some dialects the lenited form should pronounce as "ō-liv". Mostly, I hear "ō-luv" for both.