Measure words for music
一张音乐 is short for 一张音乐专辑 (a music album) or 一张音乐CD （a music CD), etc., in which cases the noun at the end of the expression is flat (LP disc, CD) and can take 张 as classifier.
一曲音乐 is a 'tuneful' of music or a 'melody-ful' of music, is also translated as 'a piece of music', I am just guessing but this could mean a piece of music which consists of at least one melody or theme, i.e. a length enough to unfold the melody or theme, or maybe longer.  
一段音乐 is a piece of music, whatever that means in English or Chinese, usually a complete piece or a complete section of a larger opus.  


@S.Rhee mentioned 场 which is a classifier for concerts if it is used in connection with music, e.g. 一场音乐会.