Hi Fulvio
We can't see your Blocks properly!
In App Inventor, right-mouse in the Blocks work area and select "Download Blocks as image".
Does the sample Project file provided by the Extension Developer work on your device?
Wave Files can be large, that might be an issue for saving. Where you can save any file depends on your Android version, the safest place being the ASD (App Specific Directory) which you will need to create the first time the App is run.