Actually, Reb Eliezer and Knaidlach, just saying yidemu with a sheva na doesn’t help. Just changing the type of sheva does not change the meaning.
The main point that actually changes the meaning from the incorrect “being similar to” to the correct “being silent” is the dagesh in the dalet, as this is to represent the missing nun from the root “nadam”. This means that you have to pronounce the dalet as double length – “yiddemu”.
Whlie we’re on the subject, the mishna berura at the beginning of siman 62 paskens that if you miss out a letter of keriyas shema, you are not yotzei the mitzvah. This is unfortunately much more common than many people think. One place many people slip up is where there is a “hei” in the middle of a word. For example the words “vihyeesem” and “lihyos” at the end of the shema. The “hei”‘s have a sheva nach and must be pronounced with an aspiration (breathing out, like a “mapik hei” at the and of a word). Many people say “viyeesem” and “liyos”, and they are not yotzei according to the mishna berura since they missed out the hei.