How to open the mac os rom?

Did a little more digging to see if was at all possible. One comment I found:

The chime is located outside the reach of the MacOS, embedded in the EPROM of the CPU board itself.

Let's assume as EPROM is correct. That makes it something you could modify (Electronically Programmable). However, the contents to such a chip are written to it before it is soldered to the system board. Trying to modify it after the fact would be about as safe to your Mac as using dynamite to open a pickle jar.

This is a awful lot of effort you're putting into for a sound you hear once, maybe twice a day. It simply isn't worth risking ruining your Mac over. If you attempt such a thing and bork the EPROM, then the only fix is to replace the system board.

You can adjust the startup sound volume, or mute it with Startup Sound.

A .rom image cannot be opened, per se. It's a binary copy of the ROM from an old Mac, and unless I've totally missed something, is all in assembly language so there's nothing that could be extracted. What is it you're attempting to accomplish that leads you to think you need to open this file to extract something?

It says the source for your ROM file is corrupt or illegal.
Obtaining ROMs is bordering on breaking a DRM, which would be considered illegal in the U.S. Have a sift through Terms of Use, and you'll see why your question is really questionable as far as posting here.
If your purpose for using the ROM is to run older Mac software, get an older Mac*:

http://www.macmaps.com/usedrefurbished.html

Emulating older Macs on newer ones using ROM readers has been very unreliable at best.

- * Links to my pages may give me compensation.

Let's start with ROM. Are you aware it means Read Only Memory? An EEPROM would be something you could change, if you have the right equipment. A ROM is a fixed information chip. So even if you could modify the ROM image, you'd have no way to burn it back to your Mac. You could also very easily brick your Mac if there were a way to do it.

google .rom file and see what you get. Where did you get this file and why do you need it?

No.