Logged in to what account?

In order to read the music on your iMac, the hard drive does need to be active, and not in a powered down state. That would spin up when you logged in to your iMac. Or, you could go in to energy settings on the iMac, and tell it never to sleep the hard drive. 

But that’s just a guess. If you could provide more specifics, the community may have a different and better answer. 

Many thanks for the reply. I suspected that might be the case. I need to think about whether to set the hard drive to never sleep or not. Maybe try and see how it goes.

Andy

Just remember, if the hard drive is asleep, the Sonos speaker can’t play any of the music stored on it. There is absolutely nothing wrong with choosing to have to spin up the hard drive when you want to listen to music, you’ll just need to be aware if it can ‘sleep’ while you’re listening, and that you need to turn it on before hand each time. My personal experience has been, in the past, that ‘wake on LAN access’ functions weren’t fast enough, but I’ve not tried that in years, so it might be different these days. And I now store my music on an NAS anyway, so I can let the Mac’s HD sleep. 

I can see what you mean about the hard drive and it going to sleep. I have heard of RAID drives, but NAS is new to me. I think it is possibly the way to go for my music and probably for storage of my photo collection which is large. You have given me something to think about for the Sonos problem and once again many thanks for your advice.

Andy