Comment 2 for bug 589307

Same error here, upgrading from karmic -> lucid all of our music on hold files went silent. Worked around this here by converting the mp3's to .sln files:

apt-get remove asterisk-mp3
apt-get install sox libsox-fmt-all mpg321
# convert mp3 to wav
mpg321 -w myfile.mp3 myfile.wav
# convert wav to sln
sox myfile.wav -t raw -r 8000 -s -c1 myfile.sln

The "-c1" up there is important if your mp3 is stereo. If you care more about filesize than sound quality you can convert them to GSM instead:

sox unknown.wav -r 8000 -c1 unknown.gsm

Best