Adjust the settings for your line in?

All settings adjusted to max.

Interesting. I’d be checking two additional things first. Check the cables you’re using to connect. Pull the out at both ends, and then reseat them. Check your source to make sure that it’s audio settings are correctly set.

What Sonos device are you having issues with?

What kind of device are you using as a source?

How are the two connected?

Knowing a little more detail might help the community narrow down what sort of advice to give you, rather than pointing to a generic FAQ, since your request for help was so generic.