aJeBosh

I'm trying to change the sound it makes when every mob dies. I've made a new resource pack of my own and inserted an OGG file, changes the sounds.js to the correct files for a zombie for example. Yet, once I actually jump into the game, every now and then it plays my file but sometimes it plays the original zombie death sound. I assumed this is only possible because its taking from the games default sound files. But..I cant for the life of me find where those files are stored. Any ideas? Thanks.