Anyone who has read Lovecraft knows that he liked to play with words, so inevitably that would have included playing with them in sound. So, we could simply assume that the well-known phrase is nothing more than a jumble of letters; or we could assume, as Lovecraft presented in many stories, that the phrase has a definite meaning in English - but coded to hide the meaning.
For example, the use of an apostrophe denotes letters that have, quite literally, been hewn away - so, Ph' could well mean 'Few'.
Go to an etymology website and look up 'ng': again, quite literally, it refers to Angles or English.
Then there is the jumble of letters, mglw': through German-Hebrew, those letters mean moglisherweise which translates as possibly/potentially.
Over time, language (its usage and pronunciation) changes. Being something of an antiquarian (among other things), Lovecraft would have known this and he clearly drew from a lot of sources for his stories.
So, for example, the Necronomicon may well have been inspired, at least in part, by the Shams al-Ma'arif; the Great Race clearly has references to Madison Grant's work; there are the remains of megalithic structures in continental North America; and Nan Madol and the structures of Pohnpei in the South Pacific clearly had an influence in such stories at The Shadow Over Innsmouth, Dagon, and The Call of Cthulhu.
Cthulhu. There was a time when the 'h' was a silent letter making, from a speech perspective, CTulu.
So I present the following:
More South Pacific and Chinese references. Chew Tulu. C'Tulu. Brine of fish.
R'lyeh. Rule. Yeh is an interesting bit. It has several different meanings. As yeh-teh it means 'small manlike animal' in old Tibetan (a place that often features in Lovecraft's stories) and is linked to the Yeti (or Mi-Go). However, it is also linked to the various names of the Abrahamic deity: Yehovah and Yaweh being two - and those names are derived from the Egyptian Moon Goddess, Iah (from which we also get Al-Iah: the true Roman script being what it is, a lower case 'l' is indistinguishable from an upper case 'I').
Ia(h), Ia(h), Shub-Niggurath. Think 'black ziggurat'. Another play on words.
And what of Yog Sothoth? Yog comes from the Turkish, in which the -g- is a "soft" sound, in many dialects closer to an English "w" and means roughly "to condense".
Which makes Yog Sothoth a condensed or agglutinated being - much as would be the case if, for example, all the Abrahamic faiths came together under the roofs of masonic lodges (of which, some of the oldest are in the Far East and Tibet).
Lastly, Lovecraft's works are focused on cosmic horror. In modern times, cosmic refers to outer space - but it used to mean worldly or of this world. It comes from the word 'kosmos' which means 'world order'.
However, all this aside, these things could simply be my way of entertaining myself and there really is nothing but a good yarn in Lovecraft's tales.