To add a custom SoundFont simply drag and drop a .sf2 file from your desktop into Paint Composer.
To change which SoundFont your song uses select one from the dropdown in the "Settings" panel.

When you "Publish to a shareable URL" the SoundFont will automatically be uploaded (if it hasn't already been) and anyone who follows the link to your song will hear it the same way you do.
Enjoy!
P.S. always feel free to share any questions, feedback, or comments. I listen to all the feedback I can to make sure that Paint Composer will make your wildest dreams come true.
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