I have a couple of ideas but I think the easiest if not the least expensive, would be to use one Arduino to read your sensors as a central controller to accessory Arduinos, one for each DFPlayer. Just have the accessory Arduinos poll an I/O for a digital signal from the central one and play the thing you want.
As for outputting each one or all together to the same amplifier, maybe some relays or even a custom audio cable, but I'd have to see all the audio gear to come up with something.