.er

The  extension is an alias for the .brstm file extension. It is used by Newer SMBW for music and sound.

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

Opening and editing ER files
You can use a  file in BrawlBox/BrawlCrate, or any other tools that can open   files to play them.

Following the same reasoning, any tool that can encode  files can create   files, since it is just an alias. It is recomended to create these using Skawo's Multi Channel BxSTM Encoder for the best quality possible and for multi-channel encoding.

Trivia
The .er extension comes from a small joke hidden path of the files using it. 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'.