Reply to Djm 400 stops output when both Master Out used but not one at a time.

Hi,
I'm new to the community and I have been trying to figure out what might be happening.
I used to own a DJM 400 which I sold to a close friend after I upgraded my gear and since he just wanted something to play with and wasn't serious into djing as I was, I hooked him up.

He used the mixer for quite some time about a year or more before a cleaning lady broke one of the volume-faders and I sent it to a repair shop that did all the work.

We never had any issues, but now I seem to have a peculiar problem. When ever we plug in to the MASTER OUT both a RCA/RCA to his speakers and use the other out to RCA/P2 (headphone jack) to record a session at his place, when ever we plug the P2 into something like a iphone to record, the output to the speakers stops and seems to be going out only to the device that the P2 is pluged.

To be quite honest the cable used to plug the mixer to the speakers (home theater) is a RCA with yellow, red and white because that was the only cable we found at the time, but I keep the color scheme (red and white) for the audio.

I have bought new RCA cables for audio only (no yellow plug) but haven't had a chance to set things up for my friend.

By any chance is this a issue due to the cable (I honestly don't believe so) or could this be some sort of short circuit that the repair shop screwed up when fixing the mixer in the first place.

Has anyone ever seen this issue? Because I used to do this all the time with no issues at his place and I can't see how can the DJM 400 stop playing to one of the Master Outputs when something else is plugged in the other??
Thanks for your attention.
Dub Hz