Sound threatening to increase loudness initially (5)
I believe the answer is:
growl
'sound' is the definition.
(I know that growl is a noise)
'increase loudness initially' is the wordplay.
'increase' becomes 'grow' (growing is a kind of increasing).
'initially' says to take the initial letters.
The first letter of 'loudness' is 'l'.
'grow'+'l'='GROWL'
'threatening to' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for growl that I've seen before include "Grumble; roar" , "Menacing animal sound" , "Hostile murmur" , "Threatening dog's noise" , "Show aggression as boxer" .)