YEP Event Chase Player Sound Issue

[Xeo]

Hello there, I've been using Yanfly's Event Chase Player Plugin in my newest project and stumbled upon an issue. I'm currently working on a Resident Evil fan game and wanted to set different sounds for each zombie, when the player gets spotted. I've input this command "this._alertSound = x" into their movement routes, but to no avail. I either get an error code, or the sound doesn't change at all. I'm getting really frustrated and can't figure it out on my own. I hope one of you can help me. Thanks!