Can't play local files on Android (Galaxy S3) phone

[androidmusicfan]

Hi,

I just signed up for Spotify with a trial Premium membership.  Spotify on my Macbook imported all my local (iTunes) files, and it can play them well on the desktop.  I downloaded the Spotify Android app, and synced a playlist of Spotify songs (i.e., non-local songs) and a playlist of local files (by using the same wi-fi network for both the phone and my laptop, and manually syncing playlists).  

On my Android phone, the Spotify (non-local) playlist works perfectly, but the playlist made of local files won't play at all.  The songs are all there, and pressing a song starts the timer as if the song was playing, but there's no sound.  I tried making this playlist available offline on the desktop app, but that made no difference.  How can I play my local files on my phone?

Thanks for your help!