Quiet passes could be deciding matches (4-4)
I believe the answer is:
play-offs
'be deciding matches' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'quiet passes' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano').
'passes' becomes 'layoffs' (I can't explain this - if you can you should give a lot more credence to this answer).
'p'+'layoffs'='PLAY-OFFS'
'could' is the link.
Can you help me to learn more?
(Other definitions for play-offs that I've seen before include "Final competitions to determine the championship" , "End of season matches between promotion contenders?" .)