MP3 has as known issue for this. I would recommend going to MP4 which is publicly available right now or else MP5 will be out in the first week of April.
See this KBA for more info:
http://www.symantec.com/docs/TECH179984
Also an older article that may or may not be relevant:
http://www.symantec.com/docs/TECH149160
Just to be safe. Test it out on a few machines that are affected by slow encryption and ensure that it works before upgrading all your production.