Heralds of Cockney birth? Changing first note, do these sound lower? (8)

'heralds of cockney birth? changing first note do' is the definition.
I can't tell whether this defines the answer.

'these sound lower?' is the wordplay.
I cannot quite understand how this works, but
'sound' could be 'bell' (bell is a kind of sound) and 'bell' is found in the answer.
'lower?' could be 'cow' (I've seen this in another clue) and 'cow' is present in the answer.
A single letter 's' remains which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?