Q: I was wondering what the easiest way is to convert music tracks to higher bit rates (ie, from 56kbps to 128kbps).
A: There is little point in converting your existing lower bit-rate tracks to higher bit-rates as you will not gain back any quality.
Audio formats like AAC and MP3 are “lossy” formats, which means that audio quality is lost when you convert them from the original source. This “lost” fidelity is gone and cannot come back simply by converting to a higher bit-rate, in much the same way that you can’t increase a picture’s quality simply by enlarging it to a higher resolution.
Converting a lower bit-rate file to a higher bit-rate will serve no purpose other than to waste space by creating a larger file with no increase in quality. The only way to get your music tracks encoded at a higher bit-rate is to re-convert them from a higher bit-rate source, ideally by re-importing them from the original CD or converting them from another lossless source such as a WAV, AIFF or FLAC format.