ls *.mp3 | perl -lne '($n=$_)=~s/(..).*/$1_my_guitar.mp3/; rename $_, 
+$n'
[download]

or

perl -e 'for (<*.mp3>){($n=$_)=~s/(..).*/$1_my_guitar.mp3/; rename $_,
+ $n}'
[download]

(both are untested)