have two roles in a Catholic Mass

1. To lead the congregation in song during "the designated moments" of the Mass, which are the following:

The Opening Hymn or song, the Gloria, the Psalm Response, the Alleluia, the Holy, Holy, Holy, the Memorial Acclamation, the Amen, the Lamb of God. 

Note that full participation in music and spoken responses is everyone's responsibility, and we are encouraged to support and encourage one another by our own presence, attentiveness and good spirit. 

2.  The second role of the cantor, choir or instruments is an offering to God of first-fruits of time and talents, and a ministry to the congregation seeking to provide appropriate music for prayer, reflection and meditation. The liturgy allows for such music before Mass, during the Offertory, during Communion and after the Dismissal.  These musical offerings draw from the Church's rich musical heritage and ministers of music are encourage by the Church to develop their talents in the service of the liturgy. Piano music, instrumental music and choral works are offered not as mere entertainment or performances, but as a form of prayer which hopefully can move others to pray as well.