Rashi on Numbers
עבדת עבדה THE SERVICE OF THE MINISTRY — (lit., a service of a service). This is the song of the Levites with accompaniment of cymbals and harps, this being the service attached to another service (the sacrificial service) (Arakhin 11a).
Rabbeinu Bahya
Siftei Chakhamim
Service for [another] service. Meaning that at the time when the Kohanim were making offerings and dealing with the service, the Levites were singing and playing on their cymbals and harps. Thus they performed a service for the service of the Kohanim.