It also does numerous other conversions, both video and audio. It's absolutely free and I use it all the time cause I love it. There's to trial runs or ads popping up.
This takes a while to set up, but you can download+install foobar2000 and Lame MP3 Encoder.
Once you get it to work, you can drag the files into foobar2000 and select all of them. Right click and do quick convert. Then you can save the files as an mp3.
FLAC is not even possible to play on iTunes