The software sox (Sound eXchange) can convert different file formats. It is interessting for translate a mp3 file to wav format and also change the rate of the file so that it become compatible of SVXLink.
Setup sox with instruction
sudo apt-get install sox
and for be usable with mp3 files
sudo apt-get install libsox-fmt-mp3
Example of conversion of a mp3 file to wav with a rate of 16 kHz
sudo sox s9plus.mp3 -r 16k /usr/share/svxlink/sounds/fr_FR/Smeter/s9plus.wav