Trying to replace volume sound in OS X to what was present before. Encountering permissions problems even as root. Help? I'm trying to get the old volume feedback noise on my MacBook. 

I have a copy of the sound file but the system is refusing to let me replace it, even as root. I found this article that also explained it step by step for terminal but even terminal is giving me problems.

I'm currently on a 2010 13" MBP with OS X 10.11 Beta 5 (15A262e). Any ideas?
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?