Hi Koans, if you were able to do this up till just recently, then something has changed in your system. You either installed another program/ or updated something, which has affected the program externally. It can't choose to not export MP3 any longer by itself.
A clean deinstall/ resinstall as admin could help, but do remember to get any updates for this version. Might be useful to know what you did/ what happened to affect it in the first place though. Cheers, M o' L.