I find myself using sentences like "that sounds great" and "it sounds like a good idea" etc. in written English. What are the formal guidelines here? Do native speakers use the verb "sounds" this way in written text, or is it just a handful of silly foreigners like myself who do?
I don't believe that there's a formal, prescriptive guideline. This is more influenced by personal preferred modes of perception than by specific language prescription. "It looks pretty good to me" would have been equally acceptable - but my preferred mode of perception is hearing rather than sight, so "sounds" comes to me more naturally.
If I were being shown some code or a design, I'd be more likely to say "looks good". If it were being described to me and I could not see it, I'd probably say "sounds good" or "sounds like it might need work".
If you want to give yourself a prescriptive rule, using "sounds" when you've received a verbal description of something and "looks" when you are looking directly at the thing in question will work. If you want to be extra vivid in a negative way, you might say, "this stinks" (a callback to Shakespeare's "something rotten in the state of Denmark"). If your perception is vague, you might say "feels good/bad", though that's more an emotional than a sensory "feel". Taste is less often used, but you might hear a good thing as "sweet", or a negative one "leaving a bad taste in one's mouth".
It's all vague approval or disapproval, so choose what feels natural to you and you'll be OK. You could use "sounds" in every case, even visual ones you're looking directly at, and no one would think it odd.