Your piece must be an original composition, not an arrangement of other
music, or even an arrangement of an earlier piece of yours. Avoid using
sound made by others if you have not received permission to use it.
(Keep in mind that a recording of old music that is in the public domain
— any Baroque piece, for example — still carries the
copyright of the people who made the recording. You would have to get
their permission to use the recording in a derivative work.)
Even if you have received permission, be careful that you’re not
using their sound as a crutch. This applies also if you incorporate a
recording of one of your acoustic pieces. If you try to use this as a
quick way out, it will show.