Short version:

Currently, it appears that when two Era 300 speakers are in a stereo pair, using the line-in for one of the speakers causes that speaker to play audio, while the other remains silent, even though the stereo pair operates as expected with other sources.

Additional background:

A few days ago I was helping a fellow Sonos owner with a setup she was trying to achieve. She has two Era 300s and a Sub, which she planned to use as a 2.1 unit. Her intended audio source was a third-party mixing board connected to the line-in of one of the Eras. In this setup, the Era physically receiving the line-in signal played audio as expected, while the other Era in the stereo pair remained silent. This was true even when the Sub was dropped from the equation. When testing other “regular” sources such as Spotify through the Sonos app, the 2.0 or 2.1 setup performed exactly as expected. All firmware was current as of 12/26.

The workaround we found was to keep the Era 300s independent, pair the Sub to one of them, and then create a group consisting of the Era+Sub and second Era. In this configuration, playing the line-in source to the group caused both Eras and the Sub to be active, although this is of course “dual mono” rather than true stereo.

She decided that dual mono might actually be better for her use case anyway, but I wanted to find out if anyone else can reproduce this, and if so determine whether it may also affect other line-in source scenarios, e.g. from a Port, or whether this issue/limitation applies only to using the line-in of an Era 300 that’s part of a stereo pair.

I’m not at this person’s home anymore, so I can’t perform further testing myself, and I suppose it’s technically possible that the mixer may have only been sending one channel of audio to the Era -- didn’t think to check that with a channel sweep test -- but I consider that highly unlikely.

Any input from other owners or feedback from Sonos -- along with a fix if appropriate! -- would be much appreciated.