How to convert .wav files to .gsm for Asterisk use

[Target]
Convert a .wav file(VIO voice greeting-VP1.wav) to .gsm

[Enviornment]
Ubuntu 7.10
Sound files of Asterisk is put under /var/lib/asterisk/sounds

[Steps]
1. Copy VP1.wav onto Desktop

2. Install sox : ae@Asterisk-PBX:/$sudo apt-get install sox

3. convert file to gsm format :
If your WAV file was in stereo, add the -c1(c one) option to convert to mono, or the output will sound very strange
ae@Asterisk-PBX:/$sox VP1.wav -r 8000 -c1 VP1.gsm resample -ql

4. put the .gsm file into Asterisk sounds folder :
ae@Asterisk-PBX:/$sudo cp VP1.gsm /var/lib/asterisk/sounds/

5. modify extensions.conf to use the new voice prompt

[reference]
http://www.voip-info.org/wiki/view/Convert+WAV+audio+files+for+use+in+Asterisk