can anyone tell me which mp3 encoder performs best at 64kbps and what arguments should be used.

Perhaps you should use mp3PRO for that bitrate? If that's not an option, at least stick with a Fraunhofer codec -- it will give you the best quality in an MP3 file at that bitrate.

Edit: Forgot to mention that you shouldn't use the Radium hacked Fraunhofer codec, because it's old and gives crappy quality compared to a lot of today's MP3 codecs.

If you're using mp3 because you need compatibility with mp3 hardware players and such, the LAME encoder or the current Fraunhofer build (probably included in Windows) will give you comparable quality in my book. Mp3PRO will give better-sounding files at the bitrate you wan't, but won't be fully compatible to be played hardware that only handles mp3. So if you're doing stuff on your PC, then I'd recommend that you go with ogg vorbis or Psytel AAC; either codec offers better sound quality at 64 kbps than does mp3. Mp3PRO and even WMA will do better than mp3 at that bitrate (but ogg or AAC is probably your best bet)

As a good discussion about Voice-Compression I can recommend the following threat:
3270 : Lame Settings For Speech?
At least, it does not exactly answer your questeion (from April ) , but it might be usefull.. .lu