Best answer by JackWright
Firstly, the more official route - buying a Sonos device with line-in. None of these options are cheap... :(
This means you can use your Play:5 (3.5mm in), Playbar/base (Digital Optical in) or Sonos Connect (RCA in) and stream the input over the Sonos network to your desired players. If you're using an analog in, expect a 70ms delay between the input and the output. Using optical in, there is a 30ms delay (one of the many reasons why the Playbar/base are better for TV use). This is purely because you're using the Play:1's as speakers over a network and not a direct connection.
Secondly, the less official route being stream your desktop audio over IP and configure a custom radio station through the desktop app. This feature doesn't work with the mobile app, however. I have done this before, though I can't remember what program I used. One caveat to this, is that the time between the action and the sound can be many seconds and isn't instant. Not ideal for general use but for using YouTube or something it's great!
In a nutshell, the Play 1's aren't really meant to be used as desktop speakers. :(
EDIT: SONOS STOP RECOMMENDING THREADS THAT ARE 4 YEARS OLD!