I'm having exactly the same problem
For example, a 123bpm track is playing around 114bpm...the pitch/key is correspondingly lower too.
What is causing this?
When I drag the track into Ableton Live or Rekordbox the tracks are at the correct speed.
When I go to check the playback speed as other users have said, it says it's at 1x....but it isn't! I've got a manual bpm counter on my phone as well...all sources and apps are telling me the music playing in Finder is slower, around 6-7%. Very odd indeed!
To the person who asked "Why are you using Finder for music storage and playback?" As the OP has said, for easier browsing, i.e., convenience. In my case the music is stored in an external hard drive, which is linked to Rekordbox. I don't use Finder for listening to music. I use it to browse for tracks either on the external hard drive, or ones located in dropbox (which appears in Finder), identify them, and if I want to use them (I'm a DJ) I will drag them into Rekordbox (or a DAW if I need to edit them).