mplayer is likely to be on your install already, and make sure you have lame, then here are two ways to do it, an easy to read version, and a short and dirty script to do it:
All wma's should be in your current directory.
Create a file called wmamp3 in your home directory (~/) containing:
current_directory=$( pwd )
for i in *.wma; do mv "$i" `echo $i | tr ' ' '_'`; done
for i in *.[Ww][Mm][Aa]; do mv "$i" `echo $i | tr '[A-Z]' '[a-z]'`; done
#Rip with Mplayer / encode with LAME
for i in *.wma ; do mplayer -vo null -vc dummy -af resample=44100 -ao pcm -waveheader $i && lame -m s audiodump.wav -o $i; done
#convert file names
for i in *.wma; do mv "$i" "`basename "$i" .wma`.mp3"; done
chmod +x ~/wmamp3 to make it executable
sudo cp ~/wmamp3 /usr/bin to pop it somewhere useful on your path
Type "wmamp3" to run your conversion.
The short and dirty version (does exactly the same as above):
for i in *.wma ; do mplayer -vo null -vc dummy -af resample=44100 -ao pcm -waveheader "$i" && lame -m j -h --vbr-new -b 160 audiodump.wav -o "`basename "$i" .wma`.mp3"; done; rm -f audiodump.wav