rename 's/\d\d\s-\s[ .A-Za-z]+-\s//' *.mp3
01 - E.S. Posthumus - Ashielf Pi.mp3 renamed as Ashielf Pi.mp3
02 - E.S. Posthumus - Oraanu Pi.mp3 renamed as Oraanu Pi.mp3
Edit: If for some reason you don't have a version of rename
installed which does the job, you can easily write the minimal version of the script yourself in Perl, and run that. This is from the Unix FAQ, and works with the regex I provided above:
#!/usr/bin/perl
#
# rename script examples from lwall:
# rename 's/\.orig$//' *.orig
# rename 'y/A-Z/a-z/ unless /^Make/' *
# rename '$_ .= ".bad"' *.f
# rename 'print "$_: "; s/foo/bar/ if <stdin> =~ /^y/i' *
$op = shift;
for (@ARGV) {
$was = $_;
eval $op;
die $@ if $@;
rename($was,$_) unless $was eq $_;
}