Asterisk supports playing MP3 files and streams with the command MP3Player.
This executes the program mpg123 with options to make the output conform to
what Asterisk expects. Unfortunately one of the options sets the scale factor
to 8192, which causes the audio to be very quiet. This option, along with every
other option except for the file, are all hard coded in the source. This is
very easy to correct though. Simply open the file apps/app_mp3.c in the
asterisk source directory using your favorite text editor. Locate the 6 lines
where mpg123 is called, search for -f or 8192. Change 8192 to something louder,
I use 32768 which is the mpg123 default. Recompile the source then reinstall
and immediately stop and start Asterisk. It might be best to do this when there
are no calls.

Using this handy command you can let callers listen to scanner or other MP3
feeds, podcasts or any other MP3 file you have or can acquire. One inventive
person downloads an MP3 recording of the current weather conditions and has it
available on their Asterisk server.

Of course this is only one solution for playing MP3 files. Another option is to
convert the MP3 file to a different format and play the resulting file. For
example I have made the album Geometry For Lovers, an album created using Apple
][ computers, available on my Asterisk server. I converted each of the 20 MP3
files into GSM files then created an extension to play each of those files.
This results is smaller files and less overhead for my poor system. I used a
simple set of commands to convert the MP3 files to WAV files using mpg123. Then
converted those to GSM files, which Asterisk seems to favor, using sox.

I have made a small script that makes it easy to convert MP3 files to GSM
files. It is called mp32gsm and can be found on my SDf Asterisk page.