Mob noises are interrupted if the sound file is triggered again during playback. This can be recreated by simply spawning a group of cows and listening. Instead of playing the sound with the original, it completely overwrites it.
Environment
Windows 8.1, java version 7 update 65 1.7.0_65-b20, intel core i7, 8G RAM, 2G GPU
Duplicate/related to MC-1538.