MacBook – change the sound of outgoing mail on the Macbook but I seem not have permissions and have yet to find a work around, any ideas

2015 macbook Pro on Catalina 10.15.7

I was hoping the simple fix here was all I needed to do: https://discussions.apple.com/thread/3239319

But it doesn't work. It seems I am unable to edit anything into the Systems folder on my Macbook's HD. I have attempted several ways of granted myself permissions but none have worked.

I Googled what to do to add permission to perform this action:

https://support.apple.com/guide/directory-utility/change-privileges-diru49b04765/mac

With this method, when I add my own username to the list and save it, then go back to check if it's still there, it's gone. It seems like it requires a domain input and not my username, so maybe that's not the right route…

https://support.apple.com/guide/mac-help/permission-files-a-mac-disk-mchlp1204/mac

Here, 'the “Ignore ownership on this volume” checkbox' does not appear for me. Maybe there is another place I can find it?

After both of these routes ended in dead ends for me I'm not sure what to try next. Anyone have an idea? I'm assuming someone has done this before, shouldn't be that hard, right?…

Edit1:

This is the message I get when I try to give myself permissions to add sound files to the HD:

The operation can’t be completed because you don’t have the necessary permission.

I used the terminal command "csrutil disable" from this YouTube video but it did not help: https://www.youtube.com/watch?v=P8L6l068q-s

Edit2:

I'm trying to add my username to the list of permissions in the "Sharing and Permissions" section of the "Get Info" window but it also gives me the message above^. First Aid on the the disks also did not work….