I have now been twice to Musée McCord: once for the permanent collection, which focuses on the near and far history of the city of Montréal, and once for the excellent featured exhibition Music - Québec: from Charlebois to Arcade Fire.
If possible, you should try to get to the music exhibition before it closes in mid-October (2014). I'm a relative newcomer to this city and province, so I learned a lot about Québec's musical heritage. Even my husband, who was born & raised here and is a big music fan, encountered a few new artists and songs.
For this exhibition, you will pick up an audioguide (included in the cost of your admission) which enables you to hear an audio clip or the full song associated with the items and commentary on display. Give yourself a good 2-3 hours (even if you don't listen to the full songs). I found the commentary helpful in understanding the way that Québec's music and culture changed so dramatically and swiftly from the Quiet Revolution to today. Very interesting, and earworms guaranteed.