So I have a Yamaha P115 and I want to be able to mix the output with a YouTube video or some other reference. I've got myself a simple 4-channel audio mixer (r/http://amzn.eu/d/aP0TXbI) to try and do this. I've connected it up to the aux out of the piano and plugged some headphones into the output but sadly I'm not getting anything through the headphones, the sound still plays through the piano speakers.
I had a look at the manual for the P115 (https://jp.yamaha.com/files/download/other_assets/8/333278/p115_en_om_b0.pdf) which states on page 23 that "When the instrument’s sound is output to an external device, first turn on the power to the instrument, then to the external device. Reverse this order when you turn the power off." but this has not helped.
I'm a bit of a noob with this stuff so apologies if I'm being stupid. Any help would be greatly appreciated.
EDIT: Just realised that if I whack the volume up on the input of the mixer I can just about hear the piano through the headphones. Do I have the wrong leads? I am using these: https://www.amazon.co.uk/gp/product/B003L6ZPQ4/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1