Why not giving them ALL they need to convert them to mp3 ? Seriously ....
Here you have a Foobar component which will allow you to play and convert them to whatever format you want :
http://www.foobar2000.org/components/view/foo_input_usf
To convert to MP3, you will also need a specific encoder, namely Lame (Lame isn't A Mp3 Encoder), here is the link
http://www.rarewares.org/mp3-lame-bundle.php . Recommending to convert in MP3 V0, which is better than MP3 320K CBR (in term of size, not in quality, because they have approx the same quality)
To convert them in lossless (FLAC mainly because its the best lossless format), check those :
http://www.rarewares.org/lossless.php
Use level compression Level 5 or Level 8 for FLAC, they are the best in term of encoding speed and size.