You may need to press harder in order to fully seat the 3.5mm plug.

Note that the Line-In is not automatically patched through to the speaker. You must select Line-In from the music source menu.

Thanks @buzz. That would make sense, but I press it as hard as I can and it does not go in. I am afraid if I press too hard, I will break the slot. Both Fives are the same way. I just don't get it. Hm...

Can you post a picture showing the connection? It would seem the jack is not fully inserted.

@Mr. T, here is the picture showing that it's not fully inserted and the type of jack I am using. Please note that it's not going in. How hard should I push is what I am wondering about. Thank you.

Can you try pressing it in a little harder?

@Mr. T, yes and I did that. It seems like this is how far it can go. What if I break it?

When I insert an aux cable into the Five Line-In, I hold the front of the speaker and push quite hard.

You’re not going to break it by fully inserting it.

@Mr. T thanks for the tips. It just feels like it's not going in. Hm...

Try the other Five? I’m wondering if the socket has something jammed in it?

@Airgetlam, I tried both. I am about to call Sonos support. Will report back shortly.

After multiple tries and failed attempts, I finally managed to push it in completely. I didn't do anything special, just pushed it harder. Weird! Thanks everyone for your replies and trying to help.