The musiconhold.conf file is used to configure different classes of music and their locations for use in Music on Hold applications. Asterisk makes use of the mpg123 application to play music to channels. You can specify arguments for a class, allowing you to use an external application to stream music either locally or over a network. Recently, native Music on Hold has been implemented, allowing Asterisk to play music without any external processes. If the file is available in the same format as the codec of the active channel, no transcoding will occur.