The new El Capitan sound volume indicator sounds very hollow and empty to me. I hate it. So I found a way to change it.
- Step 1) Disable SIP
- Reboot your computer and hold CMD + R
- Top menu bar, click on Utilities> Terminal
- Type csrutil disable and press enter
- Reboot
- Step 2) Navigate to: /System/Library/LoginPlugins/BezelServices.loginPlugin/Contents/Resources
- Rename original file to volume_original.aiff
- Copy and paste attached file into that folder, typing in your password as needed
- Step 3) Reboot
Volume File: volume.aiff
Sources:
Disable SIP: https://www.youtube.com/watch?v=7Dgg5XAYMXI
Change Notification Sound: https://www.youtube.com/watch?v=vOQOcCISgMI
File: http://forums.macrumors.com/threads/can-you-change-the-new-yosemite-volume-indicator-sound.1767911/