Death Stranding offers a great deal of equipment that can only be obtained by increasing your rank at the various UCA outposts. Others, rarer, will ask you to reconnect some shelters a little further away: this is the case of the otter hood, but also of the harmonica.
Get the harmonica
In the Eastern Region, head towards the area not connected to the UCAs to the southwest of the perimeter. Hidden in the mountains is the Musician's refuge. Enter the shelter and take the only standard command available: it is a very simple delivery mission to complete, which will ask you to wade into the nearby lake to retrieve sheet music. Fetch the two objects, bring them back to the shelter and you should connect the musician to the UCAs and thus obtain the harmonica.
Function
The harmonica allows you to soothe the BB much more effectively. To equip it, open the tab of cosmetic objects, equip yourself and then stay on round to rest. Then just press the touchpad to play a tune and rest your Bridge Baby. Be careful, it doesn't work if its gauge is already at zero.