terminology - Is there a name for this furigana convention

...where the furigana is a different "word"/"synonym" for the actual kanji in song lyrics, titles, etc.?

A couple references to this:

Is there any distinct name for this technique?

Answer

They are called 当{あ}て字{じ}.

There are two types of 当て字:

1. Ignoring the meaning of the Kanji and using them to create a sound (e.g. 珈{コー}琲{ヒー})


2. Ignoring the reading of the Kanji and using their meaning to demonstrate another sound (e.g. 衝{ショッ}撃{ク})

In this case, they are using the second type. The voices of the ghosts sound evoke 共鳴, so while you read 声, the point is that you are to imagine the sound of their actual voices as having the properties of 共鳴.

^{This is also used a lot for children's names today -- parents want to use particular Kanji, but make the reading something totally different from what the characters would normally read as. A search for 当て字名前ランキング will get you a bunch, but one example is a girl named 星{ティアラ}}