Word for speaking over others
Solution 1:
Best fit for this case :
heckling
Meaning : Shouting to interrupt a speech with which you disagree.
It has negative connotations.
Alternatively :
barracking
References :
http://www.thefreedictionary.com/heckling
http://www.thefreedictionary.com/barracking
Solution 2:
Shouting down? (If at a noisy public meeting) Drowning out? Smothering in interruptions? Norwegian has an expression, literally "to talk in someone's mouth". That fits so perfectly that it's a pity English doesn't have it – unless you want to start a fashion.