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I guessed these correctly - I get the synonym, but can someone help explain the cryptic element?
15d. Pure sound is heard (8) STRAIGHT
19d. Cover past song (7) OVERLAY

15d sounds like STRAIT (sound)

Sound = strait pronounced like straight
Past = over,+ lay which is a type of song.

Okay, so "sound" as in Plymouth sound - a stretch of water.
And Lay = a type of ballad? (looked in thesaurus) not something of which I was aware.

Bit too obscure for me.

Thanks for explaining, though, I don't like using the interweb when crosswords are involved.