I have found how to solve this "bug".
So first you need to enter this site https://audio.online-convert.com/convert-to-ogg.
Later you need to put all the custom audio files you did in the "Drop files here".
After it, choose to Change audio channels to mono. then start to convert and download.
Add it to your resource pack and there we go. Now we can custom audio files and receive a voice decrease when you go far from the source sound.
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Thanks man, that totally worked for me, you deserve an award