In its settings, make sure the volume limit is at 100%.
Check the line out level to make sure it is set for the way you use the Port. Use variable if you control the volume with the app.

All the setting are at Max. I use variable for control. My LEA professional 704 amp has noticed it's input has decreased.

Thanks for your help. 

You’ll need a line level amplifier. Here’s an example.

Thank you 

I have solved my Sonos issue with a behringer mixer. I'd like to know what happened to the Sonos output levels if anybody has any opinion. 

Had the same issue recently but audio isn’t coming out at all. Wasted an hour and a half on a call with support with no useful help. Just a bunch of diagnostics and tests.

Right, it's like we're calling the wrong number or something. This was bad on my project. I've got 4 Wiim players coming to replace these. My client needed a change.