Getting an Arduino Uno and MKR zero to communicate
Let the MKR Zero play the wav files. It has DAC on A0 and AudioZero library to play wav files from SD card. 

You can use Serial communication to command the Zero from Uno. In Uno use hardware Serial or create SoftwareSerial to send commands to Serial1 of MKR Zero. You can then test the MKR first with commands sent from Serial Monitor.