The problem is they are 24000Hz sample rate, which is not a simple 2x or 4x conversion to 44100Hz for CD audio.

http://en.wikipedia.org/wiki/SoX - SoX is one that comes to mind as being known for good sample rate conversion.
http://leute.server.de/wilde/resample.html - It has 3 possible sample rate conversion methods, with polyphase coming out as the best.

http://www.mega-nerd.com/SRC/quality.html - and another - Secret rabbit Code / libsamplerate - this may be the one SUPER is using.

Also, unless saving space is essential, I would avoid re-encoding to MP3 after resampling, it may not throw away much more than the original low bitrate/sample rate did, but if you can keep it uncompressed between conversion and burning, every little helps.