Randomly get more Estus Flasks

You can consume a Fire Keeper soul, and doing so grants you five additional units of humanity. I highly recommend you use them to upgrade your Estus Flasks however, as they are rare, and this is the only way to upgrade your flasks. If you don't upgrade them, the flasks will be of much less use later in the game since you'll (usually) have a lot more health to replenish. Here is more information on the Fire Keeper soul.

Yes, according to other players of the game, you'll hear the bell ringing if you're playing online and someone else online rings the bell.

It doesn't appear to have any effect on your game, although the interactions between games in Dark Souls is a bit mysterious and I don't have any authoritative sources confirming this.