I have never had any experience with java but have managed to get something happening. What I want is to take my custom Minecraft map (Sonic Dash) and instead of making people get a resource pack I want to add the custom .ogg files I have made and make the plugin play them.
I already have the plugin created I just don't know how to add custom sound effects and get eclipse to play them without errors.
I'm afraid this isn't possible. This is as custom sound files must be played on the client and a plugin is incapable of sending these. You must use a resource pack if you want to play any sounds apart from the existing ones. Sorry for the disappointing answer, but it is important to understand the capabilities and limits of a plugin.