The bug

Using /playsound at the eyelevel of the player does not play the sound stereo.

Note: It might be desired to play a sound at the eyelevel because it appears this is where the player hears the sound, so it would be the loudest there.

How to reproduce

1. Create a simple redstone clock which continuously activates a command block with the following command:

   /execute as @p at @s anchored eyes run playsound block.note_block.bit block @s ^ ^ ^
   

2. Rotate around
   The sound is not stereo:
   • It is louder on the left if your y-rotation is 180: /teleport @s ~ ~ ~ 180 0
   • It is louder on the right if your y-rotation is 0: /teleport @s ~ ~ ~ 0 0

Note: The direction indication of the subtitles seems to be incorrect.

Also make sure that your audio device supports stereo sound, the following commands should only play a sound on one side:

• Left: /execute as @p at @s anchored eyes run playsound block.note_block.bit block @s ^1 ^ ^
• Right: /execute as @p at @s anchored eyes run playsound block.note_block.bit block @s ^-1 ^ ^