Crossword Genius

The main challenges for sopranos, as you can hear (4,4)

Ross

I believe the answer is:

high seas

I'm a little stuck... Click here to teach me more about this clue!

'the main' is the definition.
(I've seen this before)

'challenges for sopranos as you can hear' is the wordplay.
I cannot quite see how this works, but
'as' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Another definition for high seas that I've seen is " main".)

I've seen this clue in The Times.
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