Some songs play fine, others...

Clip Sport - 8Gb.  Using MediaMonkey (MM) for syncing files and playlists.

Recently purchased two Clip Sports - one for me and one for my wife.  Synced several workout playlists from MM onto each device.  Everything worked great.

Last week I wanted to change my content, add some new songs, delete some others.  Created new playlists on MM and synced them to my Clip.  Now, some songs play fine, while others play as if on mute! I can just, just barely hear the songs, but they are absolutely playing, and the progress bar on screen is moving.  The real kicker is that there is no rhyme nor reason as to which songs play muted.  Muted songs are a mix of newly added and older songs which previously played fine.  I did delete all old content before updating the Clip with all new content.  

Thoughts?

What formats are these files? The Sandisk players seem to work best with mp3 files.

All are in MP3, between 128 and 320 bitrate

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I haven’t tried using playlists on the Clip Sport. Are some of your songs at an unusual sampling rate or bitrate? Are they fixed or variable bitrate? Are the songs that are problematic in any way different than the others? From a different source? I never used Media Monkey. 

Are you using replay gain? If so, perhaps some of those settings are messed up?