When I connect to DDJ-FLX4 with Bluetooth and press the Headphones CUE buttons (channel)/(master), there is no sound from the DDJ-FLX4 headphones.

When you are connected to DDJ-FLX4 with Bluetooth and you want to use the Headphones CUE buttons (channel)/(master), enable Split Output with the following steps:

• Connect the split cables to the mobile device.
• Connect the L side of the split cables to the speaker, and the R side to the headphones, as shown in the figure below.
• Open the rekordbox for iOS settings. Under Audio, turn on Split Output.

Split cables whose functions have been tested are as follows:

(Manufacturer / Model number / Product name)
Native Instruments / － / Traktor DJ Cable
Sound Active / 3.5-STJX2–0.5M / Exform iDJ Split Cable