What is the current source-level setting for the Line-in port and what have you selected for the Line-Out Level? If the latter is set to ‘variable’, then check your volume level on the playing Sonos room (which I presume will be your Sonos Five).

If you have chosen to not use the Fives line-in ‘autoplay’ settings, then make sure that you select the Fives ‘line-in’ as the chosen audio source to play to your chosen room from the ‘System/Music Services’ tab in the Sonos App too. (see screenshot attached).