As Sing to the Lord: Music in Divine Worship reminds us, “At other times, the liturgical season calls for a certain restraint. In Advent, for example, musical instruments should be used with moderation and should not anticipate the full joy of the Nativity of the Lord.” (114)
This restraint does not have a penitential character as is the case during Lent, but rather a “fasting” character that holds us back until it is time for full celebration.
It is important to understand that the Advent season does not imply the elimination of instruments, but rather the use of them in moderation. For example, in the case of the organ, you can simplify the registrations and perhaps bring out more strings. Familiar hymns can be sung a capella. In the case of contemporary ensembles, find ways to considerably soften instruments, or consider using acoustic instruments and hand drums or no drums. In general, find ways to give the music a lighter feel.