Thomas Kurtz

Currently the way the system is working playsound will only play the sounds that are under the delivered sound folder (under .minecraft\assets ) or sounds that have the same name and folder structure in a resourcepack as the files under minecraft\assets. see my comments in MC-19766

As the system is working right now, records are not under the sound folder so they will not play in game. I don't know if this is intended functionality or a bug.

You can force sounds to play by adding them both under .minecraft\assets\sound\xxxfolder and to a matching assets\minecraft\sound\xxxfolder. They then can play and be changed in a resourcepack - but this defeats the purpose of the resourcepack, to avoid changing delivered files and folders - but fun for experimenting.

To expand what @Steffion said the playsound command will play any sound under the sound folder in a resourcepack if that sound folder and file name already exists in the .minecraft\assets\sound\ folder structure.

Steps to duplicate.
Close Minecraft if open
Create a folder called special under the delivered folder .minecraft\assets\sound\
Copy levelup.ogg from the random folder and place it in .minecraft\assets\sound\special\ and rename it to custom.ogg
Launch 1.6.x version of Minecraft and issue /playsound special.custom @p and you will hear the levelup sound.
Close Minecraft.

Create a folder in the resourcepacks folder called sound_test and add a pack.mcmeta file in the sound_test folder.
Create the folders under sound_test\assets\minecraft\sound\special
copy the delivered sound\random file chestopen.ogg to .minecraft\resourcepacks\sound_test\assets\minecraft\sound\special and rename it to custom.ogg

Launch Minecraft and select the resource pack sound_test and /playsound special.custom @p and you will hear the chestopen sound.
Change the resource pack back to default and issue the /playsound special.custom @p and you will hear the levelup sound.
Close Minecraft

If you delete the custom file in the delivered .minecraft\assets\sound\special\ folder and launch Minecraft again you will not hear a sound by /playsound special.custom @p even with the sount_test resource pack selected.