5 “Upon a psaltery of ten strings, with a song, and upon the harp”. You have not heard of the psaltery of ten strings for the first time: it signifies the ten commandments of the Law. But we must sing upon that psaltery, and not carry it only. For even the Jews have the Law: but they carry it: they sing not....“And upon the harp.” This means, in word and deed; “with a song,” in word; “upon the harp,” in work. If you speak words alone, you have, as it were, the song only, and not the harp: if you work, and speakest not, you have the harp only. On this account both speak well and do well, if you would have the song together with the harp.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)