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Sorry, if you choose another audio device then the headphone output is not an option.
You could work around this by configuring the other sound card as your primary system audio device, then enable the PC Master output:

My problem is that i want the master audio to go to a externa soundcard.
I can choose that, but then my headphone give no sound.
So no sound when the CUE is on.
There are no other opions by headphone output
Can someone help ?
Sorry, if you choose another audio device then the headphone output is not an option.
You could work around this by configuring the other sound card as your primary system audio device, then enable the PC Master output:

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Thanks for the reply.
But I can't choose this, it is gone when I choose my external soundcard in options
Sorry, I'm not sure you understood - in your WINDOWS or MAC audio device settings, you select which audio interface you want to handle your primary system sound output. Most people have it as their built-in soundcard, but you may have another you want to use.
Within rekordbox, you would choose the DJM-400 as the audio device, then the PC Master Output is enabled.
I did this, in in deed in the options in recordbox it says oudio output DDJ400 and the soundcard. But my soundcard now is really soft in sound. It is not good for streaming.
I don't know why it is so soft.
You can open the mixer options in your system sound panel to adjust that.
You mean in my Mac options in audio options ? It is full on also
Or is there a mix options in Mac also ?
Just use your normal volume adjustments within the OS.
In my OS audio options it is full on ?
The master volume on my ddj400 isn't working now anymore
Thanks for the help
Can you show a screenshot of the rekordbox preferences with the PC MASTER enabled?
What device is "USB AUDIO CODEC" - that's not the standard Mac audio device.
Okay, so that wouldn't have volume control from the system, but it if it doesn't have any adjustment within the Audio MIDI Setup app, you'll have to adjust the outputs on the connected speakers / whatever.