What does the Torah mean when it writes "ve'Lo Ya'avod Od" and continues "Vesheires es Echav ... "?
Rashi: It means that, at the age of fifty, when the ben Levi retires from the Avodah of carrying, he continues to serve in the capacity of guarding the gates, singing in the choir 1 and loading the wagons.
Rashi (in Avos, 5:21): It means that, at the age of fifty, the ben Levi returns to advise the (younger) Levi'im and to teach them how to carry out their duties. 2
Ramban (citing the Sifri) and Rashbam: It means that he returns to the Avodah of guarding the gates and the Avodah of the b'nei Gershon
See Ramban's objections to this.
See Mishnah there.
According to the Rashbam, who does not mention singing at all, the ben Levi only continues to guard the gates. Refer also to 8:24:2:3, where he applies the same distinction to the five years prior to the age of thirty as well.
Ramban: Which was the main Avodah, but which they synchronized with the Avodah of carrying (See Ramban DH 've'Od', who elaborates further with regard to the Avodas ha'Shir).