The Bug
An empty sound means, in the sounds json, the "sounds" of it is undefined. It will be regarded valid, because it can be in the tab-complete list in the /playsound cmd. But, it displays no subtitle.
Also, /playsound a valid sound event while setting the volumn parameter 0, the subtitle will not be shown. That means, it seems impossible to show a subtitle without playing a sound.
Reproduce
/playsound minecraft:block.note_block.banjo block @s ~ ~ ~ 0 1
I can make sure I opened displaying subtitles in the option. If the volume is 0.1, the subtitle will show.
Expected Result
A subtitle would show
Linked issues
Comments 2

Thank you for your report!
We're tracking this issue in MC-192323, so this ticket is being resolved and linked as a duplicate.
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Duplicate of MC-192323.