The name of a sink can be changed but the process is dependent on the device type.
Use the Configuration Client to change the sink name in real time.
AirPlay (AirPort Express, AppleTV, etc):
Use the AirPort Utility to change the name of an AirPort Express. Use the menu system to change the name of an AppleTV.
First the name of the owning contact is used. However if this name is not set then the make and model of the device is used. (If you think this should behave differently, send us some feedback)
To change or setup the owning contact, run the "Contacts" app and edit the "ME" contact at the very top of the list.
iOS (iPhone, iPad, etc):
Change the device name in the settings.
Google Cast (Chromecast, Chromecast Audio, etc):
Use the "Google Cast" app to change the device name.
Use the Sonos controller to change the name of a speaker. (This may be built into the configuration client in the future)