Hi @Tim K.
Welcome to the Sonso community and thanks for reaching out to us. I understand you are looking for answers on how to connect your FiiO M6 to your Sonos ecosystem. Let me help you out with that.
Since you already have a Sonos Play 5 which has a line-in feature to connect any third-party music source. What we can do is use a 3.5 mm headphone jack connector (both male on both ends) and connect your FiiO M6 using the headphone port (audio out) to your Sonos PLay 5 headphone port (audio in). That should get music playing from your FiiO m^ MP3 player to your Sonos Ecosystem.
I hope this helps.
Please let us know if you still have any further questions or concerns. We are always here to help.
Thanksm