Verse 6
Let the high praises of God - Let them sing songs the most sublime, with the loudest noise consistent with harmony.
And a two-edged sword in their hand - Perhaps there is an allusion here to the manner in which the Jews were obliged to labor in rebuilding the walls of Jerusalem: "Every one with one of his hands wrought in the work, and with the other hand held a weapon," Neh 4:17.
The two-edged sword, in Hebrew, is פיפיות pipiyoth, "mouth mouths."