iTunes sync mysteriously failing in various ways
Make sure that the "Automatically install new apps" option under the "Apps" tab when in the iPhone settings in iTunes is UNchecked.

Next, from the "Summary" tab of your iPhone settings in iTunes, check the "Manually manage music and videos". 

Finally manually drag/drop the mp3 tracks you wish to add to the iPhone to the open iTunes window when it is focused on your iPhone music. This will directly add the songs without the need to sync the iPhone.