1)
Verse 18 said that these were gatekeepers. Here it says that they played instruments!
1.
'Rashi': They were changed to be singers.
2)
'Rashi' writes that gatekeepers were changed to be singers. Gatekeepers may not help singers (Rambam, Hilchos Klei ha'Mikdash 3:10)!
1.
Malbim: This was not song of Avodas ha'Mikdash. [For that], we may not have more than six lyres or less than nine harps, and only one cymbal. Here there were eight lyres, six harps and three cymbals! Rather, [the music] was merely for honor of the Aron.
3)
What is "Al Alamos"?
1.
'Rashi': Alamos are Klei Shir for certain Tehilim, e.g. "Al Alamos Shir" (Tehilim 46:1).
2.
Malbim: Perhaps they are kinds of tunes.
