Are they sung in deep voices? (3,5)

I believe the answer is:

sea songs

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'are they sung' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'deep voices?' is the wordplay.
'deep' becomes 'sea' (I've seen this before).
'voices?' becomes 'songs' (I can't justify this - if you can you should believe this answer much more).
'sea'+'songs'='SEA SONGS'

'in' acts as a link.

Can you help me to learn more?

I've seen this clue in The Globe and Mail.

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