Some songs playing at very low volume

[tcudmo]

Until today playback has been perfectly normal, but suddenly some songs are only just audiable whereas others are at the volume you would typically expect.  The volume doesn't change within songs, just from one to the next. More seem to be at the lower than the usual version. I've completely uninstalled and reinstalled but it hasnt made any difference.

Any ideas? I'm using a MacBook Air running Yosemite.

Thanks!