Moderator Note
The change happened in the new version of OpenAL-soft. Please refer to this page for further information, and potential workarounds, as well as to this comment from @unknown.
The bug
The sounds are too much related on the player's view, what causes a mono sound if a sound is applied perfectly at right or left.
Linked issues
is duplicated by 9
Comments 17

The change happened in the new version of OpenAL-soft - you should be able to get the old behavior if you manually override the openal library by adding a JVM argument -Dorg.lwjgl.openal.libname=path/to/openal_dll_or_so. openal version that lwjgl2 used is 1.15.1, which you can get here: http://kcat.strangesoft.net/openal-binaries/openal-soft-1.15.1-bin.zip

@unknown, @unknown: Just for the record. Are we talking about this issue here? https://github.com/kcat/openal-soft/issues/129

@unknown: That issue does seem to address the audio panning part, but doesn't address the strange sound filtering/quality issues that I'm getting. Now that I think of it, the filtering might be a different issue from this one.

@unknown, okay, well, maybe if you have some free minutes left, you could browse their bugtracker and see if you can find something else.

Please refrain from posting comments that are not useful to the report.
Another potential thing worth mentioning: it's not always possible to reliably tell if you're using headphones or speakers, and openal might not be configured properly as a result of that; one solution is to manually set "stereo-mode = headphones" in alsoft.ini/conf. In the future we will most likely ship with the config file, and make it somehow toggleable between headphones and speakers, but it is not really something that is possible with the current way we do things.
Best results will be produced with headphones and with hrtf, as described here: https://www.reddit.com/r/oculus/comments/1fzonq/psa_for_games_using_openal_including_minecraft/
but this is certanly not something that the game should do for you, and something that you'll have to do yourself.
Sound is a very subjective thing, and even though the newest library version with the proper configuration is my preferred setup, some people will want to keep it exactly like it was before, and that's why my original response tells how to go back to the exact way it sounded in 1.12.2.
I think that's the issue. If I'm using my desk speakers the directional sound sounds much more accurate because the sound is coming from in front of me. With headphones, it's too far to one side and there's not much center sound.
I'm using the older openal version you posted and the sound is perfect with it. Hopefully we can get speaker/headphone options in the future.
This took me longer to figure out than I'm willing to admit, but if you're trying Georgii's workaround, be sure to put quotes around your path name:
-Dorg.lwjgl.openal.libname="C:\Users\YourName\AppData\Roaming\.minecraft\openal-soft-1.15.1-bin\Win64\OpenAL32.dll"
I extracted the zip file into my .minecraft folder, but you can probably stick it anywhere.

Please add an in-game option for this.
Can confirm, also the sound quality appears to have gotten worse. Here are videos of sound in 1.12.2 and 17w43b, recorded with Windows 10's native game recorder.
I do have HRTF audio enabled via this method, but it shouldn't explain the apparent degradation in sound quality.