Trying to replace volume sound in OS X to what was present before. Encountering permissions problems even as root. Help?
The article[1] you have referenced will not work on OS X 10.11 El Capitan (beta versions included). This is due to the new System Integrity Protection (SIP) feature. SIP is intended to protect operating-level system files (in /System and other filesystem locations) from modification, even by a user with root-level privileges.

If you are committed to doing this task, you could try turning SIP off, following the article, then turning SIP back on. Instructions for disabling/enabling SIP are easily found online.

[1] Note that the article does not give any 'date posted' information, but appears to have been posted around Sept-Oct 2014?