Well, I haven't uploaded anything to GPM for a long time now, but when the service first came (early or mid 2012) out I uploaded my entire collection, and the amount of data transferred was about the same size as my audio folder.
So, either it wasn't able to match my file against their files, or their matching ability is a recent addition.
And again, a lot of my files are actually audiobooks so I can understand that not matching anything.
Originally Posted by hugh
on Sonos, will it play the song from my local network (assuming I have it) and only play from the cloud if I don't.
I'd guess that once it's uploaded, GPM will always stream and then cache the file. The reason is that Sonos wouldn't necessarily have access to the folder that was synchronized with your GPM library. Even if you have the same folder in your Sonos Library, Sonos wouldn't know that the two files are actually the same thing.
The other unfortunate thing is that given that Sonos ZPs don't have much room for caching on them, I'm pretty sure that if you were to play a couple hours of music in a loop, it would always stream the songs every time you play them. If you did that on a smartphone, it would utilize a local cache to prevent the 2nd download of the song.