Below is a simple trick to use your Sonos 5 playback as a normal AMP speaker with Line-In without any delay.
1-Reset Sonos, walkthrough initial product setup using wired network connection.
2-set delay to 75-ms and disable compression
3-connect audio jack from both ends, make sure you can hear the sound of the device connected and playing ( you will notice the delay )
4-disconnect or unplug the ethernet, no more delay, and Sonos can now be used as generic speaker.
Tip: if you have a managed or smart network switch, you can remotely control the network interface connected to Sonos, so you can switch between Sonos mode and Speaker only mode!
Hope this will help someone in the feature.