I am a performer who performs at many Jewish camps. is it OK to have background music during the 9 days?
If the music is enjoyable in its own rights, it is not permitted.
However, if the music is only incidental, such as collected sounds or slow music that contributes to the atmosphere but does not involve enjoyment in itself, there is room for leniency.
Please see here for more details.
If the show is a special event that involves joy, it shouls be restricted during the Nine Days.