Here is my setting
Mixer: Yamaha MG10Xu
speaker: yamaha hs7
piano: ROland 300NX
computer : win10
connection:
Piano connect to channel one of the mixer, speakers connect to stereo out. computer sound card connect to channel 5
I see there is signal when record, but there is no sound when playback. How to fix it ?
Assuming the audio track output is properly set to the main stereo out, it looks like you need to disable the track “record” and “monitor” functions to play back what was recorded.
Regards 
i finally know whats happenning
Then sounds comes out!!!
One more question, my Mixer has 10 inputs, (4 channel with mono Jack input). Why cubase only detect two inputs only ??? Need to upgrade software or driver ?
Because only the stereo bus is transmitted via the USB connection - read your mixer´s documentation!
so if i choose using mono to record. I should have 4 inputs.
No. One stereo input means at most two mono inputs.
As already stated only the Stereo bus comes across the USB into your computer.
so how to use channel 3-4 for recording also ?
You can use any of the inputs that you like…a mix of them will be recorded to the 2 record channels.
You send them to the stereo bus.
what about 4 physical channel records in the same time?
Which part of
only the stereo bus is transmitted via USB connection
and
No. One stereo input means at most two mono inputs.
don´t you understand?
The question has been answered 3 times already… 
Recording 4 physical channels via USB connection is not possible with your device!
The Allen & Heath Zedi10 would have been a better fit for your needs if you wanted 4 track recording in a Mixer format.
Or if a mixer is not essential there are plenty of multi input audio interface options.
