1)    Those people who recite the verses of praise out loud in a pleasant voice, and sing them, even if they do not understand what they are saying and because of this they occasionally err in the pronunciation of the words etc. have the tremendous merit that their prayers are accepted like a fragrant scent-offering, and Hashem rejoices with them. (Kaf Hachaim 51)

2)    Even on festivals we say “Mizmor shir leyom hashabbos”, as they are also called “Shabbos”. (ibid and Mishne Berurah 51)