Wondering if (a) Vedic chants are written down (I know at least in the remote past they were purely an oral tradition), and (b) where they are written down. I'm wondering if there is anywhere online (like a museum or university website perhaps, or something else) which has ancient Vedic chants written down in some sort of sheet music format or something like that. Or perhaps where might even be modern Vedic chants written down in a format.
There is nothing like ancient and modern Vedic chants. The same pattern is followed throughout generations.
Initially it was only oral tradition and they were perfected by reciting them in various methods which includes repetition of syllables or words or sentences in normal and reverse order.
Later they were penned down as text and to keep its mode of chanting the text is marked with special symbols which indicate the svara (tone/rhythm etc).
There are various authentic Vedic chanting available as audio video and text formats.
For text you can refer to websites like arya samaj as they have vedas in devanagari script and commentaries.