Murmur is a VoIP server for Mumble. It is an open-source application that is similar to programs such as Ventrilo or TeamSpeak.
The default user on a Murmur server is called 'SuperUser'. An initial password is generated for this user on first run, and you can find it by looking at the Docker logs for the Murmur container. It will be in a line that looks like
Password for 'SuperUser' set to 'something'. You can use this password to login.
It is possible to set a specific SuperUser password by using the
MURMUR_SUPW environment variable on the docker container, but due to a quirk with the way Murmur is implemented, if you set this SuperUser password variable, the container will simply update the SuperUser password and exit. As such, this variable needs to be unset to start Murmur normally.