Mappet allows loading custom sounds by placing them into .minecraft/config/mappet/sounds/, if you don't want to bother with creating a resource pack solely for sound events.

The sounds must be in .ogg format placed in config/mappet/sounds/, renaming the file extension to .ogg won't work (you need to convert to ogg using apps like Audacity). You also can use subfolders in config/mappet/sounds/ for organization, Mappet will correctly load them there as well.

Mappet's sound events named in following format: mp.sounds:SOUND_PATH_WITHOUT_EXTENSION. For example:

Sound's path relative to .minecraft	Sound event name
.minecraft/config/mappet/sounds/alex_1_1.ogg	mp.sounds:alex_1_1
.minecraft/config/mappet/sounds/fx/quests/completed.ogg	mp.sounds:fx.quests.completed
.minecraft/config/mappet/sounds/dialogue/herbert/1.ogg	mp.sounds:dialogue.herbert.1

Tutorial

Let's say we have a couple of audio files of audio files that were converted to .ogg using Audacity. Go to McLib's mod configuration panel (by pressing Ctrl + 0 in the world), click on Mappet, and click on Open sounds folder button. Over here:

After pressing that button, a file browser window should appear with opened folder where sound events go:

Move your .ogg files there, go back to Minecraft world, and press F3 + T which should freeze the game, because it will start reloading resource packs. Once resource packs will be reloaded (there will be a message in the chat, and Minecraft will unfreeze) the sound event should be available to use with /playsound command, and any place that accepts picking a sound event (like triggers).

Here is an example of adding a sound event block to a chat trigger and picking the sound event:

As you can see, all Mappet sound events start with mp.sounds: prefix.