1)
Who are "Acheihem"?
1.
Malbim: The other Leviyim.
2)
What is "Kol Avodas Mishkan Beis Elokim"?
1.
'Rashi', Malbim: They guard the gates and flay Korbanos - "veha'Leviyim Mafshitin" (Divrei ha'Yamim II, 35:11).
2.
Radak: These Roshei Avos sang in front of Mishkan Ohel Mo'ed. They did not carry on the shoulder, or any other task. Their brothers (other Leviyim) did other Avodos, like the Torah says. They carried the Mishkan 1 , until the Aron rested. Once the Aron rested, they were gatekeepers, and engaged in matters of the Mishkan and Bayis.
1
Also in Eretz Yisrael, when the Mishkan moved from place to place, Leviyim moved it. (PF)
