Forgot to mention:

Audacity is excellent for making adjustments to the sound wave with its "effects" menu, e.g. changing pitch without changing speed, amplifying the signal, cutting down crackle and hiss, etc. What it can't do - which Garageband can - is allow different settings to pan placement and volume during the track. Using both programs - Audacity first and Garageband second - can be useful in these respects.