1.16.4 - /playsound for everyone, but at a specific player
Solution 1:
Here I am, answering my own question. I found the source of the problem, and it is exactly what I commented on the other answer: when you use /playsound...@a ~ ~ ~, after the relative coords there are 3 numbers, the 1st being the volume, the 2nd being the pitch, and the 3rd being the minVolume, which I tested and found out that it is the minimum volume that can be heard out of that sound, no matter where you are, so if you set it to 0.5, even if you are 10000+ blocks away, you will hear the sound, but with the volume of 0.5.