I'm not sure exactly what you mean by "custom file".
However...if you open the Magic Music/Smartsound module you can create your audio project and export the audio only. This will give you your completed track.
If you REALLY want to be a "composer" I suggest you get Smartsound SonicFire Pro 6.
SFP6 is the big brother of Magic Music, and it's very easy to learn. Although some folks think SS/MM is pricey, you get a whole lot of bang for your buck (or pound), and on EBAY I often see highly discounted albums, as low as $12, and Smartsouund does offer sales and discounts.
Be forewarned, you might enjoy composing so much, and combined with video editing, you will become a stranger in your own home!
This message was edited 1 time. Last update was at Feb 19. 2018 08:51
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