I’ve been using the audio edition of MediaCoder for two years now and never had a problem. For freeware it’s a pretty powerful little program. I highly recommend it.
I’ve been using Free Audio Pack for years, love it, never had an issue. Well, I’ve been using the Free WMA MP3 Converter that comes in it, which does way more than just mp3s and wmas.
I’ll second @MyNewtBoobs with Free Audio Pack. It is far faster than any other converter I’ve come across, has a good range of options, and is easy to use. It is also totally free.