Stereo sounds don't become quieter when the player walks away

not just custom sounds also, as far as i know, beacon sounds, dispenser failed sounds, dropper failed sounds.

For non-custom sounds, see MC-146297.

Is this issue with custom sounds still present in 19w14a?

Are those custom sounds stereo? Since they will always play exactly on player (OpenAL limitation).

Yes, those sounds are stereo. Mono work normally, but after such a big change on the official wiki should be some information about the new functionality of sounds.

As noted in this tweet by @unknown, this is caused by the new version of OpenAL that Minecraft is using as of 1.14. Therefore resolving as working as intended.

While we are on topic of resource packs, please make sure your custom sounds have same number of channels as vanilla ones - otherwise you may have problems with volume and position (see OpenAL specification, section 5.3.4 for reason)

Rule of thumb: if you can move away from sound (i.e. it plays in world at certain position), keep it mono. If it's UI, background music or ambient sound it can be stereo.

Thank you so much for the information! This helped me greatly in figuring out where I was messing up when I made the sound files.