On PlayPlay, you can apply :
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A music fade-in: to gradually increase the sound of your music at the beginning of your video
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A music fade-out: to gradually decrease the sound of your music at the end of your video
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A fade between songs: to transition between one song and another (gradually decrease the sound of the previous one and at the same time gradually increase the sound of the next one)
To apply music fades in your video, go to the Audio tab, then to the Audio settings window, and in the Music effects section:
There you can choose which music fades to apply and their duration, by dragging the sliders between 0 and 2 seconds.
The shorter the duration, the more abrupt the fade will be. The longer the duration, the more gradual the fade will be.