I usually use Audacity to record my sound files as 16 bit 44khz WAV files.

To upload them in Asterisk, I use this small script that will create them in your destination directory as GSM files.

#! /bin/bash

FROM="/home/joe/samples"
TO="/var/lib/asterisk/sounds"

for i in $FROM/*.wav
do
echo $i
sox $i -r 8000 -c 1 $TO/$(basename $i .wav).wav resample -ql
done