There may be a problem with the specific file extension of these files. What you can try is: open Media Go--Tools--Preferences--Devices--Highlight your phone--click on Properties--Media Format--here select the Audio Format from the drop-down menu (the options vary with each device, but you can choose MP3 which your phone supports, or try a different format if this doesn't work) and untick the box that says "only encode audio when required by device".
Once this is done, delete the unplayable tracks from your phone, connect it to the PC and re-transfer them back to the phone.
