I’m reading “The Jam Maker” on Satori Reader and the opening line is this text:
小鳥たちの鳴き声がした
On the verb 鳴く、why is it 1) conjugated to the polite form 2) マス is excluded 3) 声 is pronounced ごえ and 4) how a verb is in part a subject of the subject.
鳴き声 is a noun – obviously one formed from the verb+noun originally, but it’s a word in its own right, in the same way that screwdriver is a noun originally from screw + to drive.
The answer to “why ご” is rendaku – in a compound word the first consonant in the second part often gets voiced.
You see it quite a bit with 声, probably because it’s such a common word. In kun’yomi compounds such as the one you found, it rendakus to ごえ, possibly to show that it’s one word rather than two. It stays こえ when it’s two separate words. (The same thing happens with 青空 (あおぞら) vs 青い空 (あおいそら) and other such words, although not all do.)
Another example is with “loud voice.” You can write it as 大きい声 (おおきいこえ) where it’s an adjective describing a noun. Since 声 is its own word here, it keeps its usual reading. But you can also write it 大声 (おおごえ) where it’s a stem plus a noun squished together to form a compound, so 声 experiences rendaku to reflect it being only a part of the word.
Here are a selection of other examples where you can see this in action. There are on’yomi compounds where it’s read せい (edit: or the rare しょう, here rendakud to じょう—ty to pm215 for catching that), kun’yomi compounds that are formed by sticking it to the end of a noun or verb stem (which are often used as nouns anyway) where it’s read ごえ, and phrases that are adjective + 声, where it’s read こえ. None of these are particularly common words, but you can still get an idea of how this phenomenon works.
…plus one oddball reading for good measure (じょう in 大音声), which I hadn’t seen before. I bet I’ve misread that word a time or two, especially since 音声 is usually read おんせい.