AirDrop fails to transfer files from MacBook Pro to iPhone
It appears that you are attempting to copy audio files from MacBook Pro to iPhone using AirDrop. It is not possible to transfer audio files this way. The way to get audio files from MacBook Pro to iPhone is to sync them via iTunes.

Using AirDrop, you can transfer photos, videos, locations, to an iOS devices or share a URL to open in iOS Safari. For more details, please refer to the Apple Support article, Use AirDrop to send content from your Mac.

However, it is possible to transfer an audio or any other type of file without restriction from one Mac to another using AirDrop.