This is a question best answered by lawyers and this definitely isn’t an attempt to define copyright law where you live, but in a nutshell, copyright covers implementations, not ideas.
For example, if a voice actor uses a common style for a voice recording in a cartoon and it’s an obvious concept for the contest, other voice actors are free to use that concept.
However, if they use the original voice actor’s reading of a script for the cartoon is an exact copy of someone else’s voiceover, it’s a violation of that voice actor’s copyright.
It’s the same for music contests. No one can own copyright over the idea of an electric guitar rift in octave D. Therefore, you can’t make use of an octave D guitar rift in a music contest without modifying it in some way.