Offizieller Kommentar
I have personally tested the mic functionality on the DDJ-RR with rekordbox versions 5.3 through 5.4.2, on both Mac and PC, and they have all worked without issue.
If you are having a problem, I will ask you to please file an inquiry ticket here and our rekordbox support team will be able to gather some additional information to assist you in resolving this issue.