Were over the burden bearers and directed all who did work in every kind of service: The punctuation in the Masoretic Text indicates that verse 12 is a complete verse. But then it is not clear how verse 13 is related to the previous verse. The following interpretations have been suggested:

(1) Most interpreters understand the subject of the first half of verse 13 to be “The Levites” mentioned at the end of verse 12 (so Revised Standard Version, New International Version, Traduction œcuménique de la Bible).
(2) Others, taking the words at the end of verse 12 to be parenthetical, begin a new sentence with verse 13 and add the words “Other Levites” (so Good News Translation). The sense is then that the four men named in verse 12 were entrusted with overall supervision. Under their supervision were other Levites who were in charge of the workers who carried loads and all the other workers.
(3) Others make the subject of the first half of verse 13 to be “Other Levites, who were all skilled musicians” mentioned at the end of verse 12 (so New Revised Standard Version, Contemporary English Version, New Living Translation, New Jewish Publication Society’s Tanakh, Bible en français courant).
(4) Others understand the four Levites in verse 12 to be the skillful musicians also. These same four Levites then continue as the subject of the first half of verse 13 (so New Century Version, Nouvelle Bible Segond).
(5) Since it seems strange that master musicians would have roles in supervising the workers, some translators move the clause at the end of verse 12 to the middle of verse 13 (so New English Bible/Revised English Bible, Biblia Dios Habla Hoy); for example, Revised English Bible renders these two verses as follows:

12-13 The men did their work faithfully under the supervision of Jahath and Obadiah, Levites of the line of Merari, and Zechariah and Meshullam, members of the family of Kohath. These also had control of the bearers and directed the workmen of every trade. The Levites were all skilled musicians, and some of them were secretaries, clerks, or door-keepers.

According to this interpretation, the four Levites mentioned in verse 12 are also the ones who had control over the workers who carried loads and all the other workers.

For burden bearers, see the comments on 2 Chr 2.2. New Century Version says “workers who carried loads,” and New Jewish Publication Society’s Tanakh has “porters.” All who did work in every kind of service refers to all the other workers involved in repairing the Temple. New Century Version says simply “all the other workers.”

And some of the Levites were scribes, and officials, and gatekeepers: Regarding the translation of scribes, see the comments on 1 Chr 2.55. To say that some of the Levites were scribes is probably a statement about their assignment with regard to the repair of the Temple; that is, they kept records pertaining to this job. Officials translates a Hebrew noun whose precise meaning is not clear (see the comments on 1 Chr 23.4, where it is rendered “officers”). It is used of officials whose precise duties are not always stated. It sometimes refers to officials in judicial proceedings (see 1 Chr 23.4; 26.29). Since the noun may originally have meant “scribe” or “record keeper,” Good News Translation groups scribes and officials together as those who “kept records.” An American Translation says they were “notaries.” For gatekeepers see the comments on 1 Chr 9.17.

Quoted with permission from Omanson, Roger L. and Ellington, John E. A Handbook on 1-2 Chronicles, Volume 1. (UBS Helps for Translators). Miami: UBS, 2014. For this and other handbooks for translators see here .