Sonos has two devices designed for use in a home theater setting, the Arc and the Beam. The latency inherent in any Sonos analog line in makes in relatively unsuitable for that use.

I’m assuming you’ve done all the adjustments for line in sensitivity in the controller, as outlined in the FAQ, so I’m wondering what is the source you’re feeding it? What device is it coming from, what port on that device? Have you tried a different cable?