Well, I have just tried a 20 Mb .wav file with audacity and it took 15 seconds BUT it also expects you to fill in a box for the song title, music type, artist and so on which takes more time and I can't see how to automate it. To be fair the 20 Mb file I used was one of my own recordings which did not carry any of this information (if it is possible to include it in a .wav) Thus if you have that many .wavs to recode it is going to take some time using my suggestion.
P.P.