EFC stands for Extended Function Code. It is only useful for remotes made by UEI, but it is possible to convert between EFC and other ways of describing a IR signal.

A manual search for the correct EFCs is actually pretty quick. First, though, I want to make sure that you're getting some control of the sound bar using 20752. Do you have volume responding now? If not, brute force isn't really practical, because the EFCs only change the function number and we need the correct IR protocol and device/subdevice numbers. Those are a property of the setup code.

The quick way to test EFCs is to simply go through the possibilities from 00000 to 00255. You don't need to assign the EFC using the 994 command. Just tap Setup, and enter the 5 digit EFC. The IR signal will be sent when you enter the 5th digit. Hold the 5th digit to get a repeating signal. Try it first with a known EFC from the list above, like Vol+.