There is no control accessible in the Sonos software to adjust that setting. 

Thanks for your reply.

Let me share my idea with you.

IF i play a background sound in this input did this input will be continuously up ? 

It should be, to my knowledge. I’m kind of surprised that it’s turning off in the software. The software input should be active as long as there’s something  plugged in to it. The only case in which the Sonos controller does something (switch away from that input) is when autoplay is turned on, for a device connected to the ARC input. Which you can actually turn off in the controller, and then when the line in is selected, it should stay selected until you choose to change the input. 

Come to think of it, I’m pretty sure there’s an “auto play” function associated with that Line In as well, where as it starts, it should be switching to it. The only trouble you might have with that is if the ARC input is in use as well, in which case the “auto-play” functions could be warring with each other, and the latest one to “activate” would be in play. You should be able to turn off “auto play” on either input.