The older freeware dBPowerAmp is worth a shot though I haven't used it in ages.
If you want a real solution, check out
Foobar2000. It's a media player, but it can pretty much convert anything given you have the encoder. Like for example, to encode to MP3, you just download a LAME binary and point foobar to the .exe, and that's it. Another convenient thing it will do is, it will automatically run multiple instances of the converter so it can convert more than one file at a time. For example, it might have 2 or 3 instances of LAME.exe running at once.