.er

The  file is a file format used by Newer SMBW for music and sound. It is identical to a .brstm file, just with a different file extension.

Note: To import custom music into the game, see these two articles Custom Music and Additional Music Slots

ER File Usage
You can use a  file in BrawlBox/BrawlCrate, or any other tools that can open   files.

Trivia
There is a small joke hidden in the name of these files. If you patch an ISO with NewerSMBW (highly unrecommended nowadays), these files will be placed in. If you take the file extension of these files and the folder name they're located in, then put them together, you will get 'newer'.