Hi, I have an iMac and used a DDJ200, it all worked fine.
I have now switched to the DDJ FLX4. Everything is ok, all the lights are on etc. except that there is no sound from the computer. I can load songs and play / pause using the FLX4 controller but there is no sound from the computer.
If I close DJay Pro and launch Rekordbox, everything works fine.
Please provide specific details about the Mac laptop you are using.
What macOS version are you using?
What version of djay Pro are you using? Do you have a paid djay Pro subscription?
How exactly are you connecting your computer to the FLX4 (ex. directly with USB-C cable, through a USB Hub or adapter, etc)? Please provide photos of all the cabling, USB Hubs, etc. if possible.
Is the FLX4 also connected to power or are you running it off the laptop bus power?