We need to define subtitles in the src/main/resources/assets/minecolonies/sounds.json file.
Each sound should have an appropriate translation string key and each translation key should be added to src/main/resources/assets/minecolonies/lang/en_us.lang.

For example:
(changes in bold-italic)

sounds.json

"mob.builder.female.complete": {
"category": "neutral",
"sounds": [
{
"name": "minecolonies:mob/builder/female/projectcomplete",
subtitle": "com.minecolonies.coremod.sound.builder.female.projectComplete",
"stream": false
}
},

en_us.lang

com.minecolonies.coremod.sound.builder.female.projectComplete=Builder says, "Project complete."

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Closed in favor of joint issue.