You can open local music files from your mobile device using the Audiomack app, and listen to them in your Audiomack app library. Currently, this is only available on the latest Android version of the Audiomack app.
Adding Local Music Files to your Audiomack App Library
Open your mobile device's file manager
Select the audio file you want to listen to on Audiomack
Tap the "Menu" button next to the file
Select "Open With > Audiomack"
Using Audiomack's Local File Player
Open the Audiomack app
Tap "My Library" at the bottom right corner of the screen
Tap "Offline" in the navigation menu at the top of the screen
Tap the filter icon on the top left
In the "Offline" filter menu, scroll down and select "Local File options"