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This is another of those questions that people debate endlessly. There really is no single (correct) answer - it depends on your ears, your equipment, the quality and complexity of the input sound data, etc. I think it's probably fair to say that 128 Kbps is a bit on the low side for music, but it may sound fine from some encoders for some songs under some circumstances; while 160 Kbps done by a good encoder should sound pretty good to most people under most circumstances. But I know some people will insist on higher standards. Which MP3 encoder you use really does make a difference - I personally recommend using the LAME MP3 encoder (the default in CoffeeCup CD & MP3 Ripper), or the Fraunhofer MP3 encoder.

Also note that simple speech is generally much less acoustically complex than music, and that people generally care less about the audio quality of speech, as long as it's understandable. This means that bitrates below 128 Kbps can be just fine for compressing speech, even though those bitrates are usually not even worth considering for music.