Minecraft – Why is the /playsound command not playing even though I am in range of the target selector

I'm in the process of building a multiplayer adventure map and wanted to play ambient music when players enter and as long as they remain in certain areas.
However one target selector gave me a lot of trouble over the past two days and I can't figure out what's wrong.

I made a custom soundpack and tested everything in a singleplayer world on a bit of a smaller scaling, which worked quite nicely.
For knowing how long the sound should play I'm using the scoreboard dummy variable musicRemaining, which is automatically set to 0 if a player joins the world, so they are targetable by the repeating command block playing the sound.
Players whose musicRemaining-score equals 0 and are also standing inside the rectangular prism specified in the repeating command block get to hear a nice little ambient music.
The signal then travels to a chained command block, setting the value of musicRemaining to whatever the length of the track in ticks may be (2640 in this case).
As soon as a player in the area has a musicRemaining count of >= 1, the last second repeating command block starts decreasing this value by 1 per tick. The loop runs perfectly for as long as the player remains. If a player exits the area, a repeating command block stops the music and a chained command block sets the scoreboard value to 0 so the next track can instantly start.

So far, so good, but now comes the problem I encountered in the larger scale version of the construct.
I will try to be as specific as I can be and provide all important information to solve this issue.

The first command block(repeating,always active) plays the sound to a player if the conditions are met. It holds the following command:

/playsound minecraft:ambient.custom.worldspawn music @a[dx=23,dy=10,dz=19,scores={musicRemaining=0}]

The second command block(chain, always active) is connected to the first one and executes right after it:

/scoreboard players set @a[dx=23,dy=10,dz=20,scores={musicRemaining=0}] musicRemaining 2640

The third command block(repeating, always active) runs on its own:

/scoreboard players remove @a[dx=24,dy=10,dz=19,scores={musicRemaining=1..}] musicRemaining 1

Both, the second and the third command block execute their task flawlessly in the selected area, however there seems to be a problem with the target selector in the first block, since it does not always activate when the musicRemaining-condition is met and one stands in the area, unless the player is in a radius of 14 blocks, similar as if the arguments were distance=..14.
Strange enough it still cuts the selection area as expected in any other direction.
So I'm trying to select everything in a cuboid, only get everything inside of a way smaller sphere, but still bound to the restrictions I set in place.

The colored blocks represent the area a signal is received, with the smooth blocks activating as soon as the player hitbox enters, the others only if the players cross 0.5 of the block width.

If I set the first command block to say something like "hi" and let the others go on about their business
it will fire once musicRemaining reaches 0 as it should do.
With the music command it only works in the purple area (tested by manually resetting the scoreboard variable to 0).

Any tips or advise would be really appreciated, I hope it is something trivial which I just overlooked all the time..