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. 

Thank you Bruce for the prompt reply.

I have made a mistake in above. I believe the in/out connection should be AUX instead.

And based on your response, even when I use the right connection between P5 and Mac mini, this will not be optimal as there will be an audio delay.

Correct. There is always audio delay on a Sonos analog input, a minimum of 75 ms, due to the way the system is designed to work. It can not be removed.

https://support.sonos.com/s/article/1080?language=en_US

Details here but Sonos seems to indicate you should be able to make it work.  If it doesn’t they should just come out and say it.

Adjust line in settings

https://support.sonos.com/s/article/4838?language=en_US