Obliged to try the least audible (8)

I believe the answer is:

faintest

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'least audible' is the definition.
I can't tell whether this defines the answer.

'obliged to try' is the wordplay.
'obliged' becomes 'fain' (I can't explain this - if you can you should believe this answer much more).
'to try' becomes 'test' (synonyms).
'fain'+'test'='FAINTEST'

'the' is the link.

Can you help me to learn more?

(Other definitions for faintest that I've seen before include "most dim" , "Most unenthusiastic" .)

I've seen this clue in the Sydney Morning Herald.

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