Import audio and MIDI files into GarageBand for iPhone
You can import audio and MIDI files from your computer and use them in your GarageBand song. You can add:
Audio files to an existing Audio Recorder or Amp track
MIDI files to an existing Keyboard or Drums track
Audio or MIDI files to new tracks
Audio or MIDI files to your song from iCloud Drive or your iPhone using the Files app
You can import AIFF, WAV, Apple Loops, AAC and MP3 audio files. When you import an audio file, it’s converted to a 44.1 kHz sample rate, 16-bit depth format if the original format is different. With the exception of Apple Loops, imported audio files don’t follow tempo changes you make in GarageBand.
When you import a multi-track MIDI file, GarageBand creates a new Keyboard track for each track in the MIDI file. The total number of resulting tracks cannot exceed 32 tracks. You cannot add multi-track MIDI files to cells in Live Loops.
Import audio and MIDI files from your Mac
Connect your iPhone to your Mac using a USB or USB-C cable.
Your connected iPhone appears in a Finder window sidebar under Locations.
On your Mac, select the icon for your iPhone, then click Files.
GarageBand appears in the list of apps.
Drag the audio or MIDI files you want to import from another Finder window (or from your Mac desktop) to GarageBand in the list.
In GarageBand on your iPhone, set the length of the song section to Automatic to import the entire audio or MIDI file; otherwise, only the portion of the file that fits the current song section is imported.
After importing the audio or MIDI file, you can make the song section longer, then resize the region so more of it plays.
Tap the Tracks button
to open Tracks view, then tap the Loop Browser button 
in the control bar.A message appears asking if you want to move the audio or MIDI files to the GarageBand File Transfer folder.
Tap Move Files.
The files are moved to the GarageBand File Transfer folder and are now visible in the Files tab of the Loop Browser.
To preview an audio file, tap it in the list.
Drag an audio or MIDI file left or right to show Tracks view. Align the left edge of the file with the bar or beat (on the ruler) where you want it to start playing.
A new region created from the audio or MIDI file is trimmed to the end of the current song section, unless the current song section is set to Automatic. You can make the song section longer or slow down the tempo, then resize the region so that more of it plays.
Import audio and MIDI files with the Files app
Set the length of the song section to Automatic to import the entire audio or MIDI file; otherwise, only the portion of the file that fits the current song section is imported.
After importing the file, you can make the song section longer, then resize the region so that more of it plays.
Tap the Tracks button
to open Tracks view, tap the Loop Browser button in the control bar, then tap Files.Tap “Browse items from the Files app”, then locate and tap an audio or MIDI file to import it.
To preview an audio file, tap it in the list. You can control the preview volume with the slider at the bottom of the list.
Drag an audio or MIDI file left or right to show Tracks view. Align the left edge of the file with the bar or beat (on the ruler) where you want it to start playing.
A new region created from the audio or MIDI file is trimmed to the end of the current song section, unless the current song section is set to Automatic. You can make the song section longer or slow down the tempo, then resize the region so that more of it plays.