1. Speaker 1 is going to make "a" speech about drinking alcohol.

2. Speaker 1 is going to make "the" speech about drinking alcohol.
(Do we have to use 'a' or 'the'? Are both okay? which one is commonly used?)

3. When he makes a speech, the opposite team can make an argument by standing up.

4. When he makes a speech, any speaker in the opposite team can make an argument by standing up.
(Which sentence is better? Are both okay?)

5. Question: In a debate, do we have to say,"He is going to make a speech. or He is going to make an argument?"

I like "a" better than 'the'

I like 4 better than 3 because I don't think the whole team would stand up.

In a debate, I would say argument instead of speech.