Rashi made this comment on Psalm 88:14:
a maskil: Wherever it says “maskil,” it was said through an interpreter. The prophet would set up an interpreter before him, and when he perceived a prophecy coming upon him, he would recite the prophecy to the interpreter, who would make it heard.
What might have been Rashi's source for this information? For example, I don't think I see anything about that in the Jewish Encyclopedia.