You can send a wav file to core2 using the beta version of the online UiFlow (v.1.8.2 is the latest as of today).
on UiFlow, select "VER" icon on the menu bar and select BETA, then you can use v1.8.2
From the Manager window, select "WAV" tab, and upload a wav file.
The file size must be less than 500 KB.
Maybe you also need to update your core2's firmware to v1.8.2 using the M5Burner.