Can I use "adj plus uncountable noun" between "as...as"? Do the two following sentences sound natural to you, native speakers? "Hardly ever have I heard as impressive music as this." "I can’t drink as sweet coffee as this."
Thank you very much!
The way you have phrased it is understandable, but awkward. You can use this construction with a countable noun:
Let me tell you, I have never seen as great an audience as this one.
If you replace the countable noun with an uncountable noun, it sounds less idiomatic:
* Let me tell you, I have never heard as loud applause as I heard just then.
It would be more idiomatic to rephrase it in one of two ways. First, as CocoPop suggested, you can unpack the noun phrase so that the noun comes before the first "so":
Let me tell you, I have never heard applause as loud as I heard just then.
Or you can replace the first "as" with "such". This is a somewhat more formal construction, but it sounds more idiomatic (at least in American English) than "so...so" in this context.
Let me tell you, I have never heard such loud applause as I heard just then.
One of the issues with your suggested sentence is the first half: "Hardly ever have I heard" is a somewhat stilted construction, and it accentuates the awkwardness of the rest of the sentence. "I have hardly ever heard" would be more idiomatic in spoken English and clearer and less stilted in written English.