Does the piano output only a phono level output? I’d find that most unusual, but if so, yes, Sonos is looking for a line level signal on the line in, and you’d need to add a pre-amp (or find out if there is one built in, and just needs to be switched on).

There will always be a minimum of a 75 Ms delay on any analog line in for Sonos as well. 

I don’t have a Port to check but does it have an input sensitivity control? If so try setting it around mid-range and the volume fairly low and see how things sound.

Also, if the piano line-level output is adjustable try setting it to 25% and see how things sound. If you need the Sonos volume above 50% for background level sound try 50% or more on the piano output.

With multiple volume controls in a system finding the sweet spot can take a few tweaks.

Great points. Again, as I indicated, I’d really be surprised (stunned, more like) if the piano had a phono line out level, but I’ve not bothered to look at the manual. I’ve never seen a non-phono device with a phono level output. So it would indeed make a lot of sense to look at the line in’s sensitivity on the Sonos controller’s interface. 

But @Stanley_4 hit on something else, too. Hopefully the output from the piano isn’t variable. If it is, his suggestions are excellent. Any line in, including one from Sonos, will be challenged by variable output levels. so setting something at 50% makes sense. 

I’ve only been able to find this picture of the connections on the piano (from https://princetonpianos.com/product/du1-enst-yamaha-disklavier/):

If this is the right picture it does seem you only have a headphone connection, so with a variable output.

Remember any analogue connection ads a 75ms delay, so the piano and the speakers my not be completely in sync.