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So using the smart barks pack, I associated all of the bubble's speakers with the pop sound effect, and made the matching keyword and empty string, eg: ""  

So every time any of them would speak, it would play the pop sound effect.

This is how the definitions are laid out in MVNSmartBarks.rpy:

character_voice_lines = {
    "Anonymous User": {
        "": ["sfx/pop.ogg"]
    },
     "PurplePyroGirl": {
        "": ["sfx/pop.ogg"]
    },
    "Sparkle69": {
        "": ["sfx/pop.ogg"]
    },
    "Licklad": {
        "": ["sfx/pop.ogg"]
    },
    "Fan": {
        "": ["sfx/pop.ogg"]
    },
    "": {
        "": ["sfx/pop.ogg"]
    }