Fix a few more voicemail gui issues: make sure the back button returns to the correct page, fix the redirect after deleting a voicemail. Fix the voicemail delete mp3 files or other sounds files other than wav or mp3.

git-svn-id: http://fusionpbx.googlecode.com/svn/trunk@4798 e2954ef0-caed-11de-bd98-e341d93cc3cc