I gave you a lead in chat, you shoulda scrolled up
split the audio/video, which will most likely leave you with an mp3.
edit the mp3 in your favourite audio editory prog (adobe audition should work).
join your new audio with your old video.
(this perfectly qualifies as a lead, not a step-by-step, which is what you asked for).
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- + - ° GiRLie GeeK ° - + - °
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Therell be days/When Ill stray/I may appear to be/Constantly out of reach/I give in to sin/Because I like to practise what I preach