I believe that there is a problem with this tool.
It strongly - and incorrectly? - emphasises high frequency sounds near 6KHz when converting 44KHz speech in WAV format to 8kHz AMR-NB.
So a word such as "absolutely" can sound like "abSSSSSolutely" in AMR-NB format. The "S" sounds are MUCH too strong in the AMR file.
This means that I have to hand check and the hand edit the WAV input file to Low Pass Filter the" bad" section of the input data.
Can someone at Nokia check this - I can provide a sample "bad" WAV file on request.