Where did you connect it on the Mac mini? For that matter, where on the PLAY:5? I don’t see RCA jacks on either machine.
From what I see on Apple’s website, the only audio output on the Mac Mini is a headphone output (3.5mm). You’d want to connect that to the similar (3.5mm) line in on the back of the PLAY:5. And if you have a Gen 1 PLAY:5, you’d want to be sure you weren’t accidentally using the headphone out on the Sonos.
Given that headphone outputs are variable, and not line level inputs, you’d need to play a lot with both the Sonos line in settings, and the volume output of the Mac to get a good enough signal.
And then deal with the 75ms delay minimum on an analog input on any Sonos device. There’s a reason which Sonos doesn’t market themselves as computer speakers.