While the sound itself has been disabled, I found the iconic chime is still part of the firmware itself (File GUID: 03C70B0D-67E6-5C16-8E57-312DF246A961) as part of the MacBookPro13,1 (and 13,2 or 13,3) firmware. You can find it yourself with the following steps:
- Download macOS Sierra Build 16B2657 from the appStore
- Right-click on it and select Show Package Content
- Open InstallESD.dmg in Contents -> SharedSupport
- Go to Packages -> EFIPayloads
- Copy MBP131_0205_B02_LOCKED.fd (for the MacBookPro13,1) somewhere on your machine
- Launch UEFITool and open the file and click on “Intel Image”.
- Search for “03C70B0D-67E6-5C16-8E57-312DF246A961” (GUID)
- Double-click a result
- Right-click on the Raw section and select “Extract as is” and safe it somewhere
- Strip the 1st 4 bytes in hex editor for example (everything before the ‘caff’ identifier)
- Rename your file with a ‘.caf’ extension
- Play it using Quicktime
OS Xeon 
Interesting, so I have a 2015 model MBP with the sound still enabled and now that I know the location of the raw file for the startup sound, it should theoretically be possible to flash a new firmware with an altered startup sound?
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In theory this should be possible. For current intel based macs I have however never seen anyone doing it (on the Internet). There are some articles on how to do it on PowerPC base macs and open firmware: https://www.google.com.hk/amp/www.journaldulapin.com/2013/11/15/changing-the-startup-sound-on-a-power-mac/amp/?client=safari
I personally think that just changing the raw part in the firmware is not enough as there should be some crc or similar integrity check in place.
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