Medal does not have a feature to add custom music via the editor. This is mostly due to DMCA. Music provided by Soundcloud is DMCA Free on Medal.
Step 1 - Go to your settings
Step 2 - Select Soundcloud from Social Connections
Step 2 - Grant Medal permissions to access your SoundCloud
After clicking on the Soundcloud icon, you will be prompted in a new browser window to grant access to Medal by Soundcloud.
Simply sign in to your account, if you are not already, and click on the Connect and Continue button.
You're all set!
Now that your Soundcloud account is connected to Medal, you can now add music from Soundcloud to your clips!