@Andrew Kim I think in the case of Korean under custom speech support pronunciation is not supported as per the language support page.
For the other forms that are supported you need to add the text in Korean. For example, for audio + human-labeled transcript the audio and text need to be mentioned in the transcript file referring to the audio file in supported naming format as seen below.
Here, my audio file is in folder 1 with file name 1_0003.wav and the transcript of the text is tab separated written in Korean. Additional entries can be mentioned in separate lines.
1/1_0003.wav 그는 아내를 많이 아낀다.
I hope this helps!!
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