Do we know the sounds for hieroglyphs?

Yes, this became possible after deciphering Rosetta Stone Rosetta Stone - Wikipedia which was written in 3 ancient languages Greek, Demotic and Hieroglyphic, one of them was already known Greek. read more

The fact that the Egyptians sometimes used their hieroglyphs more or less phonetically doesn't do us much good, though, because it was an abjad, where the vowels were supposed to be determined from context, and we don't know what the vowels were. We basically have to guess at the exact pronunciation of most phonetically written words. read more

Geoffrey Sampson stated that Egyptian hieroglyphs"came into existence a little after Sumerian script, and, probably [were], invented under the influence of the latter", and that it is"probable that the general idea of expressing words of a language in writing was brought to Egypt from Sumerian Mesopotamia". read more

Hieroglyphs could actually be used as an abjad (an alphabet where vowels aren't explicitly written), and often was for, for instance, foreign words. Hieroglyphs evolved into the first proto-siniac alphabet, which evolved into the Phoenician alphabet, which eventually evolved into the script we're writing in today. read more

And how we know is a fascinating tale. It won’t surprise you to learn our knowledge of how to pronounce Egyptian hieroglyphics begins with the Rosetta Stone. Discovered in 1799, the stone is a black granite-like slab on which a decree by Ptolemy V is inscribed in three languages — hieroglyphics, what’s now called demotic script, and ancient Greek. read more