If the music files are on a PC, you don’t have to use the line in, you could just stream them directly to your Sonos speakers. See this FAQ. 

Thanks for that, I did think about streaming them from the PC, although the PC is fairly old now and a bit temperamental at times. I’ll give it a go though. Looking forward to using the Era100, not so much setting it up though, we have a Sonos One  and a One SL in the lounge, paired in stereo and controlled by an Amazon Echo Dot, which was a real pain to set up, I did set just the Sonos speakers up first but we found we had to rpreall SHOUT commands, Echo dot made it much better but fiddly to sort out. I’m hoping the Eras won’t need an Echo Dot…….Thanks for the advise.

Well, you could still use the line in, with a couple of caveats. First, there would be at least a 75ms delay. And the line in is looking for a line level, stereo analog input, which most PCs only send out of their headphone jack or audio output on an audio card, which means figuring out how to fight between the volume on the PC’s output and the sensitivity of the Sonos’ input. Doing that, while a lot more effort, allows you to play anything that the PC will put out, including system sounds. You’re really getting around the fact that Sonos doesn’t make PC speakers, and paying that 75ms delay. 

After much thought I have decided not to use the MP3/ Flac music on the PC with the Era 100. There will be enough for me to listen to with Amazon Music.  Thanks for the advice.